perl assign regex match to variable

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Match regex and assign results in single line of code

I want to be able to do a regex match on a variable and assign the results to the variable itself. What is the best way to do it?

I want to essentially combine lines 2 and 3 in a single line of code:

Is there a shorter/simpler way to do this? Am I missing something?

9 Answers 9

This works because

If the /g option is not used, m// in list context returns a list consisting of the subexpressions matched by the parentheses in the pattern, i.e. , ( $1 , $2 , $3 …).

and because my($variable) = ... (note the parentheses around the scalar) supplies list context to the match.

If the pattern fails to match, $variable gets the undefined value.

• To add to the description to the list context. The OoO is equivalent to my($foo) = ("bar box" =~ /(soap|box)/); –  vol7ron Oct 17, 2012 at 16:03
• I'll also add one other example. If you want to prepend a string to the results in the same call: my($foo) = map ( "prefix_$_", @{["bar box" =~ /(soap|box)/)]} ); . You'd end up with "prefix_box" in this case. –  vol7ron Oct 17, 2012 at 18:01

Why do you want it to be shorter? Does is really matter?

If you are worried about the variable name or doing this repeatedly, wrap the thing in a subroutine and forget about it:

However you implement it, the point of the subroutine is to make it reuseable so you give a particular set of lines a short name that stands in their place.

Several other answers mention a destructive substitution:

I tend to keep the original data around, and Perl v5.14 has an /r flag that leaves the original alone and returns a new string with the replacement (instead of the count of replacements):

• I prefer this answer's use of conditional assignment to the more upvoted answer of ($variable) = $variable =~ /(find).*/ for cases where you explicitly want that conditional assignment if and only if the match works. –  stevesliva Oct 3, 2019 at 14:57

Well, you could say

• I'd rather use the variable definition separately for clarity's sake, but yes this is a good solution for quick searches. Thanks. –  user354606 Sep 6, 2010 at 15:41

You can do substitution as:

$a is now "stack"

• There is a problem with this, It will only replace the original variable if the entire string is present in the search pattern. If I were to use, say, /(\w+)over/ for matching, this wouldn't work. –  user354606 Sep 6, 2010 at 16:11
• The pattern is arbitrary, matches do not necessarily begin or end at a line or word boundary. In any case, thanks, I guess there isn't any obviously simple way to do it. My primary concern was that I have to do this often, and it looks repetitive. –  user354606 Sep 6, 2010 at 17:08
• Stumbled on this 2 year old thread via Google, and I feel this is the correct answer. Just edit the regex to fit your needs: /.*(find something).*/$1/; –  Mintx Aug 21, 2012 at 22:32
• What if you want to have two variables -- the matched pattern and the original –  QED Apr 30, 2016 at 20:22

From Perl Cookbook 2nd ed 6.1 Copying and Substituting Simultaneously

I just assumed everyone did it this way, amazed that no one gave this answer.

• This is substitution ( s/// ), not a match ( m// ). That doesn't make it not useful, but it's why it wasn't among the original answers. –  stevesliva Oct 3, 2019 at 14:49

Almost ....

You can combine the match and retrieve the matched value with a substitution.

AFAIK, You will always have to copy the value though, unless you do not care to clobber the original.

$variable1 now equals "overflow" .

Also, to amplify the accepted answer using the ternary operator to allow you to specify a default if there is no match:

• 1 Because this declares $match , this expands the most-upvoted answer as well, allowing replacement of undefined with *defaultValue* in one-line containing the variable declaration. –  stevesliva Oct 3, 2019 at 14:56

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How to use Perl regular expression and save match to a new variable

Written by Claudio Kuenzler - 0 comments

While working on a Perl script, I came across a problem when I wanted to save a part of a string into a new variable. In order to do so I decided to use a regular expression match and simply assign the match (group) to a new variable.

my $powerusage=$xml->{'POWER_SUPPLIES'}[0]->{'POWER_SUPPLY_SUMMARY'}[0]->{'PRESENT_POWER_READING'}[0]->{'VALUE'}; # At this point $powerusage value is: "167 Watts" my $powerperf = $powerusage =~ m/(\d+) [Ww]atts/i; print "power usage $powerusage ($powerperf)\n";

Basically the $powerusage variable is using a SimpleXML function to red a specific value from the XML document (saved as $xml ). This works fine and results in a string value of "167 Watts".

The $powerperf variable should only contain the actual number of wattage, without any text. I tested the following regular expression m/(\d+) [Ww]atts/i on regex101 and everything seems correct:

However instead of the expected output "power usage 167 Watts (167)", $powerperf always showed a "1" as value: "power usage 167 Watts (1)".

However when the regex is directly printed to the script's output, without saving the value to a variable, the output showed the correct value:

my $powerusage=$xml->{'POWER_SUPPLIES'}[0]->{'POWER_SUPPLY_SUMMARY'}[0]->{'PRESENT_POWER_READING'}[0]->{'VALUE'}; my $powerperf = $powerusage =~ m/(\d+) [Ww]atts/i; print "power usage $powerusage ($powerperf)\n"; print "power usage in numbers:\n"; print $powerusage =~ m/(\d+) [Ww]atts/i; print "\n";

power usage 175 Watts (1) power usage in numbers: 175

Research led to a question on stackoverflow where the same problem was discussed:

I get back a '1' as the value of $link. I assume this is because it found '1' match. But how do I save the content of the match instead?

Luckily user ikegami gave a solution where the first match can be saved into a variable by putting the variable itself into parentheses.

Note the /g to get all matches. Those can't possibly be put into a scalar. You need an array. If you just want the first match [...] note the parens (and the lack of now-useless /g). You need them to call m// in list context.

By applying this knowledge, the declaration of $powerperf was adjusted:

my $powerusage=$xml->{'POWER_SUPPLIES'}[0]->{'POWER_SUPPLY_SUMMARY'}[0]->{'PRESENT_POWER_READING'}[0]->{'VALUE'}; my ($powerperf) = $powerusage =~ m/(\d+) [Ww]atts/i; print "power usage $powerusage ($powerperf)\n";

And the output is now correct:

power usage 158 Watts (158)

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• DESCRIPTION
• Simple word matching
• Using character classes
• Matching this or that
• Grouping things and hierarchical matching
• Extracting matches
• Matching repetitions
• More matching
• Search and replace
• The split operator
• use re 'strict'
• Acknowledgments

perlrequick - Perl regular expressions quick start

# DESCRIPTION

This page covers the very basics of understanding, creating and using regular expressions ('regexes') in Perl.

# The Guide

This page assumes you already know things, like what a "pattern" is, and the basic syntax of using them. If you don't, see perlretut .

# Simple word matching

The simplest regex is simply a word, or more generally, a string of characters. A regex consisting of a word matches any string that contains that word:

In this statement, World is a regex and the // enclosing /World/ tells Perl to search a string for a match. The operator =~ associates the string with the regex match and produces a true value if the regex matched, or false if the regex did not match. In our case, World matches the second word in "Hello World" , so the expression is true. This idea has several variations.

Expressions like this are useful in conditionals:

The sense of the match can be reversed by using !~ operator:

The literal string in the regex can be replaced by a variable:

If you're matching against $_ , the $_ =~ part can be omitted:

Finally, the // default delimiters for a match can be changed to arbitrary delimiters by putting an 'm' out front:

Regexes must match a part of the string exactly in order for the statement to be true:

Perl will always match at the earliest possible point in the string:

Not all characters can be used 'as is' in a match. Some characters, called metacharacters , are considered special, and reserved for use in regex notation. The metacharacters are

A metacharacter can be matched literally by putting a backslash before it:

In the last regex, the forward slash '/' is also backslashed, because it is used to delimit the regex.

Most of the metacharacters aren't always special, and other characters (such as the ones delimiting the pattern) become special under various circumstances. This can be confusing and lead to unexpected results. use re 'strict' can notify you of potential pitfalls.

Non-printable ASCII characters are represented by escape sequences . Common examples are \t for a tab, \n for a newline, and \r for a carriage return. Arbitrary bytes are represented by octal escape sequences, e.g., \033 , or hexadecimal escape sequences, e.g., \x1B :

Regexes are treated mostly as double-quoted strings, so variable substitution works:

With all of the regexes above, if the regex matched anywhere in the string, it was considered a match. To specify where it should match, we would use the anchor metacharacters ^ and $ . The anchor ^ means match at the beginning of the string and the anchor $ means match at the end of the string, or before a newline at the end of the string. Some examples:

# Using character classes

A character class allows a set of possible characters, rather than just a single character, to match at a particular point in a regex. There are a number of different types of character classes, but usually when people use this term, they are referring to the type described in this section, which are technically called "Bracketed character classes", because they are denoted by brackets [...] , with the set of characters to be possibly matched inside. But we'll drop the "bracketed" below to correspond with common usage. Here are some examples of (bracketed) character classes:

In the last statement, even though 'c' is the first character in the class, the earliest point at which the regex can match is 'a' .

The last example shows a match with an 'i' modifier , which makes the match case-insensitive.

Character classes also have ordinary and special characters, but the sets of ordinary and special characters inside a character class are different than those outside a character class. The special characters for a character class are -]\^$ and are matched using an escape:

The special character '-' acts as a range operator within character classes, so that the unwieldy [0123456789] and [abc...xyz] become the svelte [0-9] and [a-z] :

If '-' is the first or last character in a character class, it is treated as an ordinary character.

The special character ^ in the first position of a character class denotes a negated character class , which matches any character but those in the brackets. Both [...] and [^...] must match a character, or the match fails. Then

Perl has several abbreviations for common character classes. (These definitions are those that Perl uses in ASCII-safe mode with the /a modifier. Otherwise they could match many more non-ASCII Unicode characters as well. See "Backslash sequences" in perlrecharclass for details.)

\d is a digit and represents

\s is a whitespace character and represents

\w is a word character (alphanumeric or _) and represents

\D is a negated \d; it represents any character but a digit

\S is a negated \s; it represents any non-whitespace character

\W is a negated \w; it represents any non-word character

The period '.' matches any character but "\n"

The \d\s\w\D\S\W abbreviations can be used both inside and outside of character classes. Here are some in use:

The word anchor \b matches a boundary between a word character and a non-word character \w\W or \W\w :

In the last example, the end of the string is considered a word boundary.

For natural language processing (so that, for example, apostrophes are included in words), use instead \b{wb}

# Matching this or that

We can match different character strings with the alternation metacharacter '|' . To match dog or cat , we form the regex dog|cat . As before, Perl will try to match the regex at the earliest possible point in the string. At each character position, Perl will first try to match the first alternative, dog . If dog doesn't match, Perl will then try the next alternative, cat . If cat doesn't match either, then the match fails and Perl moves to the next position in the string. Some examples:

Even though dog is the first alternative in the second regex, cat is able to match earlier in the string.

At a given character position, the first alternative that allows the regex match to succeed will be the one that matches. Here, all the alternatives match at the first string position, so the first matches.

# Grouping things and hierarchical matching

The grouping metacharacters () allow a part of a regex to be treated as a single unit. Parts of a regex are grouped by enclosing them in parentheses. The regex house(cat|keeper) means match house followed by either cat or keeper . Some more examples are

# Extracting matches

The grouping metacharacters () also allow the extraction of the parts of a string that matched. For each grouping, the part that matched inside goes into the special variables $1 , $2 , etc. They can be used just as ordinary variables:

In list context, a match /regex/ with groupings will return the list of matched values ($1,$2,...) . So we could rewrite it as

If the groupings in a regex are nested, $1 gets the group with the leftmost opening parenthesis, $2 the next opening parenthesis, etc. For example, here is a complex regex and the matching variables indicated below it:

Associated with the matching variables $1 , $2 , ... are the backreferences \g1 , \g2 , ... Backreferences are matching variables that can be used inside a regex:

$1 , $2 , ... should only be used outside of a regex, and \g1 , \g2 , ... only inside a regex.

# Matching repetitions

The quantifier metacharacters ? , * , + , and {} allow us to determine the number of repeats of a portion of a regex we consider to be a match. Quantifiers are put immediately after the character, character class, or grouping that we want to specify. They have the following meanings:

a? = match 'a' 1 or 0 times

a* = match 'a' 0 or more times, i.e., any number of times

a+ = match 'a' 1 or more times, i.e., at least once

a{n,m} = match at least n times, but not more than m times.

a{n,} = match at least n or more times

a{,n} = match n times or fewer

a{n} = match exactly n times

Here are some examples:

These quantifiers will try to match as much of the string as possible, while still allowing the regex to match. So we have

The first quantifier .* grabs as much of the string as possible while still having the regex match. The second quantifier .* has no string left to it, so it matches 0 times.

# More matching

There are a few more things you might want to know about matching operators. The global modifier /g allows the matching operator to match within a string as many times as possible. In scalar context, successive matches against a string will have /g jump from match to match, keeping track of position in the string as it goes along. You can get or set the position with the pos() function. For example,

A failed match or changing the target string resets the position. If you don't want the position reset after failure to match, add the /c , as in /regex/gc .

In list context, /g returns a list of matched groupings, or if there are no groupings, a list of matches to the whole regex. So

# Search and replace

Search and replace is performed using s/regex/replacement/modifiers . The replacement is a Perl double-quoted string that replaces in the string whatever is matched with the regex . The operator =~ is also used here to associate a string with s/// . If matching against $_ , the $_ =~ can be dropped. If there is a match, s/// returns the number of substitutions made; otherwise it returns false. Here are a few examples:

With the s/// operator, the matched variables $1 , $2 , etc. are immediately available for use in the replacement expression. With the global modifier, s///g will search and replace all occurrences of the regex in the string:

The non-destructive modifier s///r causes the result of the substitution to be returned instead of modifying $_ (or whatever variable the substitute was bound to with =~ ):

The evaluation modifier s///e wraps an eval{...} around the replacement string and the evaluated result is substituted for the matched substring. Some examples:

The last example shows that s/// can use other delimiters, such as s!!! and s{}{} , and even s{}// . If single quotes are used s''' , then the regex and replacement are treated as single-quoted strings.

# The split operator

split /regex/, string splits string into a list of substrings and returns that list. The regex determines the character sequence that string is split with respect to. For example, to split a string into words, use

To extract a comma-delimited list of numbers, use

If the empty regex // is used, the string is split into individual characters. If the regex has groupings, then the list produced contains the matched substrings from the groupings as well:

Since the first character of $x matched the regex, split prepended an empty initial element to the list.

# use re 'strict'

New in v5.22, this applies stricter rules than otherwise when compiling regular expression patterns. It can find things that, while legal, may not be what you intended.

See 'strict' in re .

This is just a quick start guide. For a more in-depth tutorial on regexes, see perlretut and for the reference page, see perlre .

# AUTHOR AND COPYRIGHT

Copyright (c) 2000 Mark Kvale All rights reserved.

This document may be distributed under the same terms as Perl itself.

# Acknowledgments

The author would like to thank Mark-Jason Dominus, Tom Christiansen, Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful comments.

Perldoc Browser is maintained by Dan Book ( DBOOK ). Please contact him via the GitHub issue tracker or email regarding any issues with the site itself, search, or rendering of documentation.

The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the Perl issue tracker , the mailing list , or IRC to report any issues with the contents or format of the documentation.

perlretut - Perl regular expressions tutorial

DESCRIPTION

This page provides a basic tutorial on understanding, creating and using regular expressions in Perl. It serves as a complement to the reference page on regular expressions perlre . Regular expressions are an integral part of the m// , s/// , qr// and split operators and so this tutorial also overlaps with Regexp Quote-Like Operators in perlop and split in perlfunc .

Perl is widely renowned for excellence in text processing, and regular expressions are one of the big factors behind this fame. Perl regular expressions display an efficiency and flexibility unknown in most other computer languages. Mastering even the basics of regular expressions will allow you to manipulate text with surprising ease.

What is a regular expression? A regular expression is simply a string that describes a pattern. Patterns are in common use these days; examples are the patterns typed into a search engine to find web pages and the patterns used to list files in a directory, e.g., ls *.txt or dir *.* . In Perl, the patterns described by regular expressions are used to search strings, extract desired parts of strings, and to do search and replace operations.

Regular expressions have the undeserved reputation of being abstract and difficult to understand. Regular expressions are constructed using simple concepts like conditionals and loops and are no more difficult to understand than the corresponding if conditionals and while loops in the Perl language itself. In fact, the main challenge in learning regular expressions is just getting used to the terse notation used to express these concepts.

This tutorial flattens the learning curve by discussing regular expression concepts, along with their notation, one at a time and with many examples. The first part of the tutorial will progress from the simplest word searches to the basic regular expression concepts. If you master the first part, you will have all the tools needed to solve about 98% of your needs. The second part of the tutorial is for those comfortable with the basics and hungry for more power tools. It discusses the more advanced regular expression operators and introduces the latest cutting edge innovations in 5.6.0.

A note: to save time, 'regular expression' is often abbreviated as regexp or regex. Regexp is a more natural abbreviation than regex, but is harder to pronounce. The Perl pod documentation is evenly split on regexp vs regex; in Perl, there is more than one way to abbreviate it. We'll use regexp in this tutorial.

Part 1: The basics

Simple word matching.

The simplest regexp is simply a word, or more generally, a string of characters. A regexp consisting of a word matches any string that contains that word:

What is this perl statement all about? "Hello World" is a simple double quoted string. World is the regular expression and the // enclosing /World/ tells perl to search a string for a match. The operator =~ associates the string with the regexp match and produces a true value if the regexp matched, or false if the regexp did not match. In our case, World matches the second word in "Hello World" , so the expression is true. Expressions like this are useful in conditionals:

There are useful variations on this theme. The sense of the match can be reversed by using !~ operator:

The literal string in the regexp can be replaced by a variable:

If you're matching against the special default variable $_ , the $_ =~ part can be omitted:

And finally, the // default delimiters for a match can be changed to arbitrary delimiters by putting an 'm' out front:

/World/ , m!World! , and m{World} all represent the same thing. When, e.g., "" is used as a delimiter, the forward slash '/' becomes an ordinary character and can be used in a regexp without trouble.

Let's consider how different regexps would match "Hello World" :

The first regexp world doesn't match because regexps are case-sensitive. The second regexp matches because the substring 'o W'   occurs in the string "Hello World"  . The space character ' ' is treated like any other character in a regexp and is needed to match in this case. The lack of a space character is the reason the third regexp 'oW' doesn't match. The fourth regexp 'World ' doesn't match because there is a space at the end of the regexp, but not at the end of the string. The lesson here is that regexps must match a part of the string exactly in order for the statement to be true.

If a regexp matches in more than one place in the string, perl will always match at the earliest possible point in the string:

With respect to character matching, there are a few more points you need to know about. First of all, not all characters can be used 'as is' in a match. Some characters, called metacharacters , are reserved for use in regexp notation. The metacharacters are

The significance of each of these will be explained in the rest of the tutorial, but for now, it is important only to know that a metacharacter can be matched by putting a backslash before it:

In the last regexp, the forward slash '/' is also backslashed, because it is used to delimit the regexp. This can lead to LTS (leaning toothpick syndrome), however, and it is often more readable to change delimiters.

The backslash character '\' is a metacharacter itself and needs to be backslashed:

In addition to the metacharacters, there are some ASCII characters which don't have printable character equivalents and are instead represented by escape sequences . Common examples are \t for a tab, \n for a newline, \r for a carriage return and \a for a bell. If your string is better thought of as a sequence of arbitrary bytes, the octal escape sequence, e.g., \033 , or hexadecimal escape sequence, e.g., \x1B may be a more natural representation for your bytes. Here are some examples of escapes:

If you've been around Perl a while, all this talk of escape sequences may seem familiar. Similar escape sequences are used in double-quoted strings and in fact the regexps in Perl are mostly treated as double-quoted strings. This means that variables can be used in regexps as well. Just like double-quoted strings, the values of the variables in the regexp will be substituted in before the regexp is evaluated for matching purposes. So we have:

So far, so good. With the knowledge above you can already perform searches with just about any literal string regexp you can dream up. Here is a very simple emulation of the Unix grep program:

This program is easy to understand. #!/usr/bin/perl is the standard way to invoke a perl program from the shell. $regexp = shift;   saves the first command line argument as the regexp to be used, leaving the rest of the command line arguments to be treated as files. while (<>)   loops over all the lines in all the files. For each line, print if /$regexp/;   prints the line if the regexp matches the line. In this line, both print and /$regexp/ use the default variable $_ implicitly.

With all of the regexps above, if the regexp matched anywhere in the string, it was considered a match. Sometimes, however, we'd like to specify where in the string the regexp should try to match. To do this, we would use the anchor metacharacters ^ and $ . The anchor ^ means match at the beginning of the string and the anchor $ means match at the end of the string, or before a newline at the end of the string. Here is how they are used:

The second regexp doesn't match because ^ constrains keeper to match only at the beginning of the string, but "housekeeper" has keeper starting in the middle. The third regexp does match, since the $ constrains keeper to match only at the end of the string.

When both ^ and $ are used at the same time, the regexp has to match both the beginning and the end of the string, i.e., the regexp matches the whole string. Consider

The first regexp doesn't match because the string has more to it than keep . Since the second regexp is exactly the string, it matches. Using both ^ and $ in a regexp forces the complete string to match, so it gives you complete control over which strings match and which don't. Suppose you are looking for a fellow named bert, off in a string by himself:

Of course, in the case of a literal string, one could just as easily use the string equivalence $string eq 'bert'   and it would be more efficient. The ^...$ regexp really becomes useful when we add in the more powerful regexp tools below.

Using character classes

Although one can already do quite a lot with the literal string regexps above, we've only scratched the surface of regular expression technology. In this and subsequent sections we will introduce regexp concepts (and associated metacharacter notations) that will allow a regexp to not just represent a single character sequence, but a whole class of them.

One such concept is that of a character class . A character class allows a set of possible characters, rather than just a single character, to match at a particular point in a regexp. Character classes are denoted by brackets [...] , with the set of characters to be possibly matched inside. Here are some examples:

In the last statement, even though 'c' is the first character in the class, 'a' matches because the first character position in the string is the earliest point at which the regexp can match.

This regexp displays a common task: perform a case-insensitive match. Perl provides away of avoiding all those brackets by simply appending an 'i' to the end of the match. Then /[yY][eE][sS]/; can be rewritten as /yes/i; . The 'i' stands for case-insensitive and is an example of a modifier of the matching operation. We will meet other modifiers later in the tutorial.

We saw in the section above that there were ordinary characters, which represented themselves, and special characters, which needed a backslash \ to represent themselves. The same is true in a character class, but the sets of ordinary and special characters inside a character class are different than those outside a character class. The special characters for a character class are -]\^$ . ] is special because it denotes the end of a character class. $ is special because it denotes a scalar variable. \ is special because it is used in escape sequences, just like above. Here is how the special characters ]$\ are handled:

The last two are a little tricky. in [\$x] , the backslash protects the dollar sign, so the character class has two members $ and x . In [\\$x] , the backslash is protected, so $x is treated as a variable and substituted in double quote fashion.

The special character '-' acts as a range operator within character classes, so that a contiguous set of characters can be written as a range. With ranges, the unwieldy [0123456789] and [abc...xyz] become the svelte [0-9] and [a-z] . Some examples are

If '-' is the first or last character in a character class, it is treated as an ordinary character; [-ab] , [ab-] and [a\-b] are all equivalent.

The special character ^ in the first position of a character class denotes a negated character class , which matches any character but those in the brackets. Both [...] and [^...] must match a character, or the match fails. Then

Now, even [0-9] can be a bother the write multiple times, so in the interest of saving keystrokes and making regexps more readable, Perl has several abbreviations for common character classes:

\d is a digit and represents [0-9]

\s is a whitespace character and represents [\ \t\r\n\f]

\w is a word character (alphanumeric or _) and represents [0-9a-zA-Z_]

\D is a negated \d; it represents any character but a digit [^0-9]

\S is a negated \s; it represents any non-whitespace character [^\s]

\W is a negated \w; it represents any non-word character [^\w]

The period '.' matches any character but "\n"

The \d\s\w\D\S\W abbreviations can be used both inside and outside of character classes. Here are some in use:

Because a period is a metacharacter, it needs to be escaped to match as an ordinary period. Because, for example, \d and \w are sets of characters, it is incorrect to think of [^\d\w] as [\D\W] ; in fact [^\d\w] is the same as [^\w] , which is the same as [\W] . Think DeMorgan's laws.

An anchor useful in basic regexps is the word anchor   \b . This matches a boundary between a word character and a non-word character \w\W or \W\w :

Note in the last example, the end of the string is considered a word boundary.

You might wonder why '.' matches everything but "\n" - why not every character? The reason is that often one is matching against lines and would like to ignore the newline characters. For instance, while the string "\n" represents one line, we would like to think of as empty. Then

This behavior is convenient, because we usually want to ignore newlines when we count and match characters in a line. Sometimes, however, we want to keep track of newlines. We might even want ^ and $ to anchor at the beginning and end of lines within the string, rather than just the beginning and end of the string. Perl allows us to choose between ignoring and paying attention to newlines by using the //s and //m modifiers. //s and //m stand for single line and multi-line and they determine whether a string is to be treated as one continuous string, or as a set of lines. The two modifiers affect two aspects of how the regexp is interpreted: 1) how the '.' character class is defined, and 2) where the anchors ^ and $ are able to match. Here are the four possible combinations:

no modifiers (//): Default behavior. '.' matches any character except "\n" . ^ matches only at the beginning of the string and $ matches only at the end or before a newline at the end.

s modifier (//s): Treat string as a single long line. '.' matches any character, even "\n" . ^ matches only at the beginning of the string and $ matches only at the end or before a newline at the end.

m modifier (//m): Treat string as a set of multiple lines. '.' matches any character except "\n" . ^ and $ are able to match at the start or end of any line within the string.

both s and m modifiers (//sm): Treat string as a single long line, but detect multiple lines. '.' matches any character, even "\n" . ^ and $ , however, are able to match at the start or end of any line within the string.

Here are examples of //s and //m in action:

Most of the time, the default behavior is what is wanted, but //s and //m are occasionally very useful. If //m is being used, the start of the string can still be matched with \A and the end of string can still be matched with the anchors \Z (matches both the end and the newline before, like $ ), and \z (matches only the end):

We now know how to create choices among classes of characters in a regexp. What about choices among words or character strings? Such choices are described in the next section.

Matching this or that

Sometimes we would like to our regexp to be able to match different possible words or character strings. This is accomplished by using the alternation metacharacter | . To match dog or cat , we form the regexp dog|cat . As before, perl will try to match the regexp at the earliest possible point in the string. At each character position, perl will first try to match the first alternative, dog . If dog doesn't match, perl will then try the next alternative, cat . If cat doesn't match either, then the match fails and perl moves to the next position in the string. Some examples:

Even though dog is the first alternative in the second regexp, cat is able to match earlier in the string.

Here, all the alternatives match at the first string position, so the first alternative is the one that matches. If some of the alternatives are truncations of the others, put the longest ones first to give them a chance to match.

The last example points out that character classes are like alternations of characters. At a given character position, the first alternative that allows the regexp match to succeed will be the one that matches.

Grouping things and hierarchical matching

Alternation allows a regexp to choose among alternatives, but by itself it unsatisfying. The reason is that each alternative is a whole regexp, but sometime we want alternatives for just part of a regexp. For instance, suppose we want to search for housecats or housekeepers. The regexp housecat|housekeeper fits the bill, but is inefficient because we had to type house twice. It would be nice to have parts of the regexp be constant, like house , and some parts have alternatives, like cat|keeper .

The grouping metacharacters () solve this problem. Grouping allows parts of a regexp to be treated as a single unit. Parts of a regexp are grouped by enclosing them in parentheses. Thus we could solve the housecat|housekeeper by forming the regexp as house(cat|keeper) . The regexp house(cat|keeper) means match house followed by either cat or keeper . Some more examples are

Alternations behave the same way in groups as out of them: at a given string position, the leftmost alternative that allows the regexp to match is taken. So in the last example at the first string position, "20" matches the second alternative, but there is nothing left over to match the next two digits \d\d . So perl moves on to the next alternative, which is the null alternative and that works, since "20" is two digits.

The process of trying one alternative, seeing if it matches, and moving on to the next alternative if it doesn't, is called backtracking . The term 'backtracking' comes from the idea that matching a regexp is like a walk in the woods. Successfully matching a regexp is like arriving at a destination. There are many possible trailheads, one for each string position, and each one is tried in order, left to right. From each trailhead there may be many paths, some of which get you there, and some which are dead ends. When you walk along a trail and hit a dead end, you have to backtrack along the trail to an earlier point to try another trail. If you hit your destination, you stop immediately and forget about trying all the other trails. You are persistent, and only if you have tried all the trails from all the trailheads and not arrived at your destination, do you declare failure. To be concrete, here is a step-by-step analysis of what perl does when it tries to match the regexp

Start with the first letter in the string 'a'.

Try the first alternative in the first group 'abd'.

Match 'a' followed by 'b'. So far so good.

'd' in the regexp doesn't match 'c' in the string - a dead end. So backtrack two characters and pick the second alternative in the first group 'abc'.

Match 'a' followed by 'b' followed by 'c'. We are on a roll and have satisfied the first group. Set $1 to 'abc'.

Move on to the second group and pick the first alternative 'df'.

Match the 'd'.

'f' in the regexp doesn't match 'e' in the string, so a dead end. Backtrack one character and pick the second alternative in the second group 'd'.

'd' matches. The second grouping is satisfied, so set $2 to 'd'.

We are at the end of the regexp, so we are done! We have matched 'abcd' out of the string "abcde".

There are a couple of things to note about this analysis. First, the third alternative in the second group 'de' also allows a match, but we stopped before we got to it - at a given character position, leftmost wins. Second, we were able to get a match at the first character position of the string 'a'. If there were no matches at the first position, perl would move to the second character position 'b' and attempt the match all over again. Only when all possible paths at all possible character positions have been exhausted does perl give up and declare $string =~ /(abd|abc)(df|d|de)/;   to be false.

Even with all this work, regexp matching happens remarkably fast. To speed things up, during compilation stage, perl compiles the regexp into a compact sequence of opcodes that can often fit inside a processor cache. When the code is executed, these opcodes can then run at full throttle and search very quickly.

Extracting matches

The grouping metacharacters () also serve another completely different function: they allow the extraction of the parts of a string that matched. This is very useful to find out what matched and for text processing in general. For each grouping, the part that matched inside goes into the special variables $1 , $2 , etc. They can be used just as ordinary variables:

Now, we know that in scalar context, $time =~ /(\d\d):(\d\d):(\d\d)/   returns a true or false value. In list context, however, it returns the list of matched values ($1,$2,$3) . So we could write the code more compactly as

If the groupings in a regexp are nested, $1 gets the group with the leftmost opening parenthesis, $2 the next opening parenthesis, etc. For example, here is a complex regexp and the matching variables indicated below it:

so that if the regexp matched, e.g., $2 would contain 'cd' or 'ef'. For convenience, perl sets $+ to the string held by the highest numbered $1 , $2 , ... that got assigned (and, somewhat related, $^N to the value of the $1 , $2 , ... most-recently assigned; i.e. the $1 , $2 , ... associated with the rightmost closing parenthesis used in the match).

Closely associated with the matching variables $1 , $2 , ... are the backreferences \1 , \2 , ... . Backreferences are simply matching variables that can be used inside a regexp. This is a really nice feature - what matches later in a regexp can depend on what matched earlier in the regexp. Suppose we wanted to look for doubled words in text, like 'the the'. The following regexp finds all 3-letter doubles with a space in between:

The grouping assigns a value to \1, so that the same 3 letter sequence is used for both parts. Here are some words with repeated parts:

The regexp has a single grouping which considers 4-letter combinations, then 3-letter combinations, etc. and uses \1 to look for a repeat. Although $1 and \1 represent the same thing, care should be taken to use matched variables $1 , $2 , ... only outside a regexp and backreferences \1 , \2 , ... only inside a regexp; not doing so may lead to surprising and/or undefined results.

In addition to what was matched, Perl 5.6.0 also provides the positions of what was matched with the @- and @+ arrays. $-[0] is the position of the start of the entire match and $+[0] is the position of the end. Similarly, $-[n] is the position of the start of the $n match and $+[n] is the position of the end. If $n is undefined, so are $-[n] and $+[n] . Then this code

Even if there are no groupings in a regexp, it is still possible to find out what exactly matched in a string. If you use them, perl will set $` to the part of the string before the match, will set $& to the part of the string that matched, and will set $' to the part of the string after the match. An example:

In the second match, $` = ''   because the regexp matched at the first character position in the string and stopped, it never saw the second 'the'. It is important to note that using $` and $' slows down regexp matching quite a bit, and $& slows it down to a lesser extent, because if they are used in one regexp in a program, they are generated for <all> regexps in the program. So if raw performance is a goal of your application, they should be avoided. If you need them, use @- and @+ instead:

Matching repetitions

The examples in the previous section display an annoying weakness. We were only matching 3-letter words, or syllables of 4 letters or less. We'd like to be able to match words or syllables of any length, without writing out tedious alternatives like \w\w\w\w|\w\w\w|\w\w|\w .

This is exactly the problem the quantifier metacharacters ? , * , + , and {} were created for. They allow us to determine the number of repeats of a portion of a regexp we consider to be a match. Quantifiers are put immediately after the character, character class, or grouping that we want to specify. They have the following meanings:

a? = match 'a' 1 or 0 times

a* = match 'a' 0 or more times, i.e., any number of times

a+ = match 'a' 1 or more times, i.e., at least once

a{n,m} = match at least n times, but not more than m times.

a{n,} = match at least n or more times

a{n} = match exactly n times

Here are some examples:

For all of these quantifiers, perl will try to match as much of the string as possible, while still allowing the regexp to succeed. Thus with /a?.../ , perl will first try to match the regexp with the a present; if that fails, perl will try to match the regexp without the a present. For the quantifier * , we get the following:

Which is what we might expect, the match finds the only cat in the string and locks onto it. Consider, however, this regexp:

One might initially guess that perl would find the at in cat and stop there, but that wouldn't give the longest possible string to the first quantifier .* . Instead, the first quantifier .* grabs as much of the string as possible while still having the regexp match. In this example, that means having the at sequence with the final at in the string. The other important principle illustrated here is that when there are two or more elements in a regexp, the leftmost quantifier, if there is one, gets to grab as much the string as possible, leaving the rest of the regexp to fight over scraps. Thus in our example, the first quantifier .* grabs most of the string, while the second quantifier .* gets the empty string. Quantifiers that grab as much of the string as possible are called maximal match or greedy quantifiers.

When a regexp can match a string in several different ways, we can use the principles above to predict which way the regexp will match:

Principle 0: Taken as a whole, any regexp will be matched at the earliest possible position in the string.

Principle 1: In an alternation a|b|c... , the leftmost alternative that allows a match for the whole regexp will be the one used.

Principle 2: The maximal matching quantifiers ? , * , + and {n,m} will in general match as much of the string as possible while still allowing the whole regexp to match.

Principle 3: If there are two or more elements in a regexp, the leftmost greedy quantifier, if any, will match as much of the string as possible while still allowing the whole regexp to match. The next leftmost greedy quantifier, if any, will try to match as much of the string remaining available to it as possible, while still allowing the whole regexp to match. And so on, until all the regexp elements are satisfied.

As we have seen above, Principle 0 overrides the others - the regexp will be matched as early as possible, with the other principles determining how the regexp matches at that earliest character position.

Here is an example of these principles in action:

This regexp matches at the earliest string position, 'T' . One might think that e , being leftmost in the alternation, would be matched, but r produces the longest string in the first quantifier.

Here, The earliest possible match is at the first 'm' in programming . m{1,2} is the first quantifier, so it gets to match a maximal mm .

Here, the regexp matches at the start of the string. The first quantifier .* grabs as much as possible, leaving just a single 'm' for the second quantifier m{1,2} .

Here, .? eats its maximal one character at the earliest possible position in the string, 'a' in programming , leaving m{1,2} the opportunity to match both m 's. Finally,

because it can match zero copies of 'X' at the beginning of the string. If you definitely want to match at least one 'X' , use X+ , not X* .

Sometimes greed is not good. At times, we would like quantifiers to match a minimal piece of string, rather than a maximal piece. For this purpose, Larry Wall created the minimal match   or non-greedy quantifiers ?? , *? , +? , and {}? . These are the usual quantifiers with a ? appended to them. They have the following meanings:

a?? = match 'a' 0 or 1 times. Try 0 first, then 1.

a*? = match 'a' 0 or more times, i.e., any number of times, but as few times as possible

a+? = match 'a' 1 or more times, i.e., at least once, but as few times as possible

a{n,m}? = match at least n times, not more than m times, as few times as possible

a{n,}? = match at least n times, but as few times as possible

a{n}? = match exactly n times. Because we match exactly n times, a{n}? is equivalent to a{n} and is just there for notational consistency.

Let's look at the example above, but with minimal quantifiers:

The minimal string that will allow both the start of the string ^ and the alternation to match is Th , with the alternation e|r matching e . The second quantifier .* is free to gobble up the rest of the string.

The first string position that this regexp can match is at the first 'm' in programming . At this position, the minimal m{1,2}? matches just one 'm' . Although the second quantifier .*? would prefer to match no characters, it is constrained by the end-of-string anchor $ to match the rest of the string.

In this regexp, you might expect the first minimal quantifier .*? to match the empty string, because it is not constrained by a ^ anchor to match the beginning of the word. Principle 0 applies here, however. Because it is possible for the whole regexp to match at the start of the string, it will match at the start of the string. Thus the first quantifier has to match everything up to the first m . The second minimal quantifier matches just one m and the third quantifier matches the rest of the string.

Just as in the previous regexp, the first quantifier .?? can match earliest at position 'a' , so it does. The second quantifier is greedy, so it matches mm , and the third matches the rest of the string.

We can modify principle 3 above to take into account non-greedy quantifiers:

Principle 3: If there are two or more elements in a regexp, the leftmost greedy (non-greedy) quantifier, if any, will match as much (little) of the string as possible while still allowing the whole regexp to match. The next leftmost greedy (non-greedy) quantifier, if any, will try to match as much (little) of the string remaining available to it as possible, while still allowing the whole regexp to match. And so on, until all the regexp elements are satisfied.

Just like alternation, quantifiers are also susceptible to backtracking. Here is a step-by-step analysis of the example

Start with the first letter in the string 't'.

The first quantifier '.*' starts out by matching the whole string 'the cat in the hat'.

'a' in the regexp element 'at' doesn't match the end of the string. Backtrack one character.

'a' in the regexp element 'at' still doesn't match the last letter of the string 't', so backtrack one more character.

Now we can match the 'a' and the 't'.

Move on to the third element '.*'. Since we are at the end of the string and '.*' can match 0 times, assign it the empty string.

We are done!

Most of the time, all this moving forward and backtracking happens quickly and searching is fast. There are some pathological regexps, however, whose execution time exponentially grows with the size of the string. A typical structure that blows up in your face is of the form

The problem is the nested indeterminate quantifiers. There are many different ways of partitioning a string of length n between the + and * : one repetition with b+ of length n, two repetitions with the first b+ length k and the second with length n-k, m repetitions whose bits add up to length n, etc. In fact there are an exponential number of ways to partition a string as a function of length. A regexp may get lucky and match early in the process, but if there is no match, perl will try every possibility before giving up. So be careful with nested * 's, {n,m} 's, and + 's. The book Mastering regular expressions by Jeffrey Friedl gives a wonderful discussion of this and other efficiency issues.

Building a regexp

At this point, we have all the basic regexp concepts covered, so let's give a more involved example of a regular expression. We will build a regexp that matches numbers.

The first task in building a regexp is to decide what we want to match and what we want to exclude. In our case, we want to match both integers and floating point numbers and we want to reject any string that isn't a number.

The next task is to break the problem down into smaller problems that are easily converted into a regexp.

The simplest case is integers. These consist of a sequence of digits, with an optional sign in front. The digits we can represent with \d+ and the sign can be matched with [+-] . Thus the integer regexp is

A floating point number potentially has a sign, an integral part, a decimal point, a fractional part, and an exponent. One or more of these parts is optional, so we need to check out the different possibilities. Floating point numbers which are in proper form include 123., 0.345, .34, -1e6, and 25.4E-72. As with integers, the sign out front is completely optional and can be matched by [+-]? . We can see that if there is no exponent, floating point numbers must have a decimal point, otherwise they are integers. We might be tempted to model these with \d*\.\d* , but this would also match just a single decimal point, which is not a number. So the three cases of floating point number sans exponent are

These can be combined into a single regexp with a three-way alternation:

In this alternation, it is important to put '\d+\.\d+' before '\d+\.' . If '\d+\.' were first, the regexp would happily match that and ignore the fractional part of the number.

Now consider floating point numbers with exponents. The key observation here is that both integers and numbers with decimal points are allowed in front of an exponent. Then exponents, like the overall sign, are independent of whether we are matching numbers with or without decimal points, and can be 'decoupled' from the mantissa. The overall form of the regexp now becomes clear:

The exponent is an e or E , followed by an integer. So the exponent regexp is

Putting all the parts together, we get a regexp that matches numbers:

Long regexps like this may impress your friends, but can be hard to decipher. In complex situations like this, the //x modifier for a match is invaluable. It allows one to put nearly arbitrary whitespace and comments into a regexp without affecting their meaning. Using it, we can rewrite our 'extended' regexp in the more pleasing form

If whitespace is mostly irrelevant, how does one include space characters in an extended regexp? The answer is to backslash it '\ '   or put it in a character class [ ]  . The same thing goes for pound signs, use \# or [#] . For instance, Perl allows a space between the sign and the mantissa/integer, and we could add this to our regexp as follows:

In this form, it is easier to see a way to simplify the alternation. Alternatives 1, 2, and 4 all start with \d+ , so it could be factored out:

or written in the compact form,

This is our final regexp. To recap, we built a regexp by

specifying the task in detail,

breaking down the problem into smaller parts,

translating the small parts into regexps,

combining the regexps,

and optimizing the final combined regexp.

These are also the typical steps involved in writing a computer program. This makes perfect sense, because regular expressions are essentially programs written a little computer language that specifies patterns.

Using regular expressions in Perl

The last topic of Part 1 briefly covers how regexps are used in Perl programs. Where do they fit into Perl syntax?

We have already introduced the matching operator in its default /regexp/ and arbitrary delimiter m!regexp! forms. We have used the binding operator =~ and its negation !~ to test for string matches. Associated with the matching operator, we have discussed the single line //s , multi-line //m , case-insensitive //i and extended //x modifiers.

There are a few more things you might want to know about matching operators. First, we pointed out earlier that variables in regexps are substituted before the regexp is evaluated:

This will print any lines containing the word Seuss . It is not as efficient as it could be, however, because perl has to re-evaluate $pattern each time through the loop. If $pattern won't be changing over the lifetime of the script, we can add the //o modifier, which directs perl to only perform variable substitutions once:

If you change $pattern after the first substitution happens, perl will ignore it. If you don't want any substitutions at all, use the special delimiter m'' :

m'' acts like single quotes on a regexp; all other m delimiters act like double quotes. If the regexp evaluates to the empty string, the regexp in the last successful match is used instead. So we have

The final two modifiers //g and //c concern multiple matches. The modifier //g stands for global matching and allows the matching operator to match within a string as many times as possible. In scalar context, successive invocations against a string will have ` //g jump from match to match, keeping track of position in the string as it goes along. You can get or set the position with the pos() function.

The use of //g is shown in the following example. Suppose we have a string that consists of words separated by spaces. If we know how many words there are in advance, we could extract the words using groupings:

But what if we had an indeterminate number of words? This is the sort of task //g was made for. To extract all words, form the simple regexp (\w+) and loop over all matches with /(\w+)/g :

A failed match or changing the target string resets the position. If you don't want the position reset after failure to match, add the //c , as in /regexp/gc . The current position in the string is associated with the string, not the regexp. This means that different strings have different positions and their respective positions can be set or read independently.

In list context, //g returns a list of matched groupings, or if there are no groupings, a list of matches to the whole regexp. So if we wanted just the words, we could use

Closely associated with the //g modifier is the \G anchor. The \G anchor matches at the point where the previous //g match left off. \G allows us to easily do context-sensitive matching:

The combination of //g and \G allows us to process the string a bit at a time and use arbitrary Perl logic to decide what to do next. Currently, the \G anchor is only fully supported when used to anchor to the start of the pattern.

\G is also invaluable in processing fixed length records with regexps. Suppose we have a snippet of coding region DNA, encoded as base pair letters ATCGTTGAAT... and we want to find all the stop codons TGA . In a coding region, codons are 3-letter sequences, so we can think of the DNA snippet as a sequence of 3-letter records. The naive regexp

doesn't work; it may match a TGA , but there is no guarantee that the match is aligned with codon boundaries, e.g., the substring GTT GAA   gives a match. A better solution is

which prints

Position 18 is good, but position 23 is bogus. What happened?

The answer is that our regexp works well until we get past the last real match. Then the regexp will fail to match a synchronized TGA and start stepping ahead one character position at a time, not what we want. The solution is to use \G to anchor the match to the codon alignment:

This prints

which is the correct answer. This example illustrates that it is important not only to match what is desired, but to reject what is not desired.

search and replace

Regular expressions also play a big role in search and replace operations in Perl. Search and replace is accomplished with the s/// operator. The general form is s/regexp/replacement/modifiers , with everything we know about regexps and modifiers applying in this case as well. The replacement is a Perl double quoted string that replaces in the string whatever is matched with the regexp . The operator =~ is also used here to associate a string with s/// . If matching against $_ , the $_ =~   can be dropped. If there is a match, s/// returns the number of substitutions made, otherwise it returns false. Here are a few examples:

In the last example, the whole string was matched, but only the part inside the single quotes was grouped. With the s/// operator, the matched variables $1 , $2 , etc. are immediately available for use in the replacement expression, so we use $1 to replace the quoted string with just what was quoted. With the global modifier, s///g will search and replace all occurrences of the regexp in the string:

If you prefer 'regex' over 'regexp' in this tutorial, you could use the following program to replace it:

In simple_replace we used the s///g modifier to replace all occurrences of the regexp on each line and the s///o modifier to compile the regexp only once. As with simple_grep , both the print and the s/$regexp/$replacement/go use $_ implicitly.

A modifier available specifically to search and replace is the s///e evaluation modifier. s///e wraps an eval{...} around the replacement string and the evaluated result is substituted for the matched substring. s///e is useful if you need to do a bit of computation in the process of replacing text. This example counts character frequencies in a line:

As with the match m// operator, s/// can use other delimiters, such as s!!! and s{}{} , and even s{}// . If single quotes are used s''' , then the regexp and replacement are treated as single quoted strings and there are no substitutions. s/// in list context returns the same thing as in scalar context, i.e., the number of matches.

The split operator

The split function can also optionally use a matching operator m// to split a string. split /regexp/, string, limit splits string into a list of substrings and returns that list. The regexp is used to match the character sequence that the string is split with respect to. The limit , if present, constrains splitting into no more than limit number of strings. For example, to split a string into words, use

If the empty regexp // is used, the regexp always matches and the string is split into individual characters. If the regexp has groupings, then list produced contains the matched substrings from the groupings as well. For instance,

Since the first character of $x matched the regexp, split prepended an empty initial element to the list.

If you have read this far, congratulations! You now have all the basic tools needed to use regular expressions to solve a wide range of text processing problems. If this is your first time through the tutorial, why not stop here and play around with regexps a while... Part 2 concerns the more esoteric aspects of regular expressions and those concepts certainly aren't needed right at the start.

Part 2: Power tools

OK, you know the basics of regexps and you want to know more. If matching regular expressions is analogous to a walk in the woods, then the tools discussed in Part 1 are analogous to topo maps and a compass, basic tools we use all the time. Most of the tools in part 2 are analogous to flare guns and satellite phones. They aren't used too often on a hike, but when we are stuck, they can be invaluable.

What follows are the more advanced, less used, or sometimes esoteric capabilities of perl regexps. In Part 2, we will assume you are comfortable with the basics and concentrate on the new features.

More on characters, strings, and character classes

There are a number of escape sequences and character classes that we haven't covered yet.

There are several escape sequences that convert characters or strings between upper and lower case. \l and \u convert the next character to lower or upper case, respectively:

\L and \U converts a whole substring, delimited by \L or \U and \E , to lower or upper case:

If there is no \E , case is converted until the end of the string. The regexps \L\u$word or \u\L$word convert the first character of $word to uppercase and the rest of the characters to lowercase.

Control characters can be escaped with \c , so that a control-Z character would be matched with \cZ . The escape sequence \Q ... \E quotes, or protects most non-alphabetic characters. For instance,

It does not protect $ or @ , so that variables can still be substituted.

With the advent of 5.6.0, perl regexps can handle more than just the standard ASCII character set. Perl now supports Unicode , a standard for encoding the character sets from many of the world's written languages. Unicode does this by allowing characters to be more than one byte wide. Perl uses the UTF-8 encoding, in which ASCII characters are still encoded as one byte, but characters greater than chr(127) may be stored as two or more bytes.

What does this mean for regexps? Well, regexp users don't need to know much about perl's internal representation of strings. But they do need to know 1) how to represent Unicode characters in a regexp and 2) when a matching operation will treat the string to be searched as a sequence of bytes (the old way) or as a sequence of Unicode characters (the new way). The answer to 1) is that Unicode characters greater than chr(127) may be represented using the \x{hex} notation, with hex a hexadecimal integer:

Unicode characters in the range of 128-255 use two hexadecimal digits with braces: \x{ab} . Note that this is different than \xab , which is just a hexadecimal byte with no Unicode significance.

NOTE : in Perl 5.6.0 it used to be that one needed to say use utf8 to use any Unicode features. This is no more the case: for almost all Unicode processing, the explicit utf8 pragma is not needed. (The only case where it matters is if your Perl script is in Unicode and encoded in UTF-8, then an explicit use utf8 is needed.)

Figuring out the hexadecimal sequence of a Unicode character you want or deciphering someone else's hexadecimal Unicode regexp is about as much fun as programming in machine code. So another way to specify Unicode characters is to use the named character   escape sequence \N{name} . name is a name for the Unicode character, as specified in the Unicode standard. For instance, if we wanted to represent or match the astrological sign for the planet Mercury, we could use

One can also use short names or restrict names to a certain alphabet:

A list of full names is found in the file Names.txt in the lib/perl5/5.X.X/unicore directory.

The answer to requirement 2), as of 5.6.0, is that if a regexp contains Unicode characters, the string is searched as a sequence of Unicode characters. Otherwise, the string is searched as a sequence of bytes. If the string is being searched as a sequence of Unicode characters, but matching a single byte is required, we can use the \C escape sequence. \C is a character class akin to . except that it matches any byte 0-255. So

The last regexp matches, but is dangerous because the string character position is no longer synchronized to the string byte position. This generates the warning 'Malformed UTF-8 character'. The \C is best used for matching the binary data in strings with binary data intermixed with Unicode characters.

Let us now discuss the rest of the character classes. Just as with Unicode characters, there are named Unicode character classes represented by the \p{name} escape sequence. Closely associated is the \P{name} character class, which is the negation of the \p{name} class. For example, to match lower and uppercase characters,

Here is the association between some Perl named classes and the traditional Unicode classes:

You can also use the official Unicode class names with the \p and \P , like \p{L} for Unicode 'letters', or \p{Lu} for uppercase letters, or \P{Nd} for non-digits. If a name is just one letter, the braces can be dropped. For instance, \pM is the character class of Unicode 'marks', for example accent marks. For the full list see perlunicode .

The Unicode has also been separated into various sets of characters which you can test with \p{In...} (in) and \P{In...} (not in), for example \p{Latin} , \p{Greek} , or \P{Katakana} . For the full list see perlunicode .

\X is an abbreviation for a character class sequence that includes the Unicode 'combining character sequences'. A 'combining character sequence' is a base character followed by any number of combining characters. An example of a combining character is an accent. Using the Unicode full names, e.g., A + COMBINING RING   is a combining character sequence with base character A and combining character COMBINING RING  , which translates in Danish to A with the circle atop it, as in the word Angstrom. \X is equivalent to \PM\pM*} , i.e., a non-mark followed by one or more marks.

For the full and latest information about Unicode see the latest Unicode standard, or the Unicode Consortium's website http://www.unicode.org/

As if all those classes weren't enough, Perl also defines POSIX style character classes. These have the form [:name:] , with name the name of the POSIX class. The POSIX classes are alpha , alnum , ascii , cntrl , digit , graph , lower , print , punct , space , upper , and xdigit , and two extensions, word (a Perl extension to match \w ), and blank (a GNU extension). If utf8 is being used, then these classes are defined the same as their corresponding perl Unicode classes: [:upper:] is the same as \p{IsUpper} , etc. The POSIX character classes, however, don't require using utf8 . The [:digit:] , [:word:] , and [:space:] correspond to the familiar \d , \w , and \s character classes. To negate a POSIX class, put a ^ in front of the name, so that, e.g., [:^digit:] corresponds to \D and under utf8 , \P{IsDigit} . The Unicode and POSIX character classes can be used just like \d , with the exception that POSIX character classes can only be used inside of a character class:

Whew! That is all the rest of the characters and character classes.

Compiling and saving regular expressions

In Part 1 we discussed the //o modifier, which compiles a regexp just once. This suggests that a compiled regexp is some data structure that can be stored once and used again and again. The regexp quote qr// does exactly that: qr/string/ compiles the string as a regexp and transforms the result into a form that can be assigned to a variable:

Then $reg can be used as a regexp:

$reg can also be interpolated into a larger regexp:

As with the matching operator, the regexp quote can use different delimiters, e.g., qr!! , qr{} and qr~~ . The single quote delimiters qr'' prevent any interpolation from taking place.

Pre-compiled regexps are useful for creating dynamic matches that don't need to be recompiled each time they are encountered. Using pre-compiled regexps, simple_grep program can be expanded into a program that matches multiple patterns:

Storing pre-compiled regexps in an array @compiled allows us to simply loop through the regexps without any recompilation, thus gaining flexibility without sacrificing speed.

Embedding comments and modifiers in a regular expression

Starting with this section, we will be discussing Perl's set of extended patterns . These are extensions to the traditional regular expression syntax that provide powerful new tools for pattern matching. We have already seen extensions in the form of the minimal matching constructs ?? , *? , +? , {n,m}? , and {n,}? . The rest of the extensions below have the form (?char...) , where the char is a character that determines the type of extension.

The first extension is an embedded comment (?#text) . This embeds a comment into the regular expression without affecting its meaning. The comment should not have any closing parentheses in the text. An example is

This style of commenting has been largely superseded by the raw, freeform commenting that is allowed with the //x modifier.

The modifiers //i , //m , //s , and //x can also embedded in a regexp using (?i) , (?m) , (?s) , and (?x) . For instance,

Embedded modifiers can have two important advantages over the usual modifiers. Embedded modifiers allow a custom set of modifiers to each regexp pattern. This is great for matching an array of regexps that must have different modifiers:

The second advantage is that embedded modifiers only affect the regexp inside the group the embedded modifier is contained in. So grouping can be used to localize the modifier's effects:

Embedded modifiers can also turn off any modifiers already present by using, e.g., (?-i) . Modifiers can also be combined into a single expression, e.g., (?s-i) turns on single line mode and turns off case insensitivity.

Non-capturing groupings

We noted in Part 1 that groupings () had two distinct functions: 1) group regexp elements together as a single unit, and 2) extract, or capture, substrings that matched the regexp in the grouping. Non-capturing groupings, denoted by (?:regexp) , allow the regexp to be treated as a single unit, but don't extract substrings or set matching variables $1 , etc. Both capturing and non-capturing groupings are allowed to co-exist in the same regexp. Because there is no extraction, non-capturing groupings are faster than capturing groupings. Non-capturing groupings are also handy for choosing exactly which parts of a regexp are to be extracted to matching variables:

Non-capturing groupings are also useful for removing nuisance elements gathered from a split operation:

Non-capturing groupings may also have embedded modifiers: (?i-m:regexp) is a non-capturing grouping that matches regexp case insensitively and turns off multi-line mode.

Looking ahead and looking behind

This section concerns the lookahead and lookbehind assertions. First, a little background.

In Perl regular expressions, most regexp elements 'eat up' a certain amount of string when they match. For instance, the regexp element [abc}] eats up one character of the string when it matches, in the sense that perl moves to the next character position in the string after the match. There are some elements, however, that don't eat up characters (advance the character position) if they match. The examples we have seen so far are the anchors. The anchor ^ matches the beginning of the line, but doesn't eat any characters. Similarly, the word boundary anchor \b matches, e.g., if the character to the left is a word character and the character to the right is a non-word character, but it doesn't eat up any characters itself. Anchors are examples of 'zero-width assertions'. Zero-width, because they consume no characters, and assertions, because they test some property of the string. In the context of our walk in the woods analogy to regexp matching, most regexp elements move us along a trail, but anchors have us stop a moment and check our surroundings. If the local environment checks out, we can proceed forward. But if the local environment doesn't satisfy us, we must backtrack.

Checking the environment entails either looking ahead on the trail, looking behind, or both. ^ looks behind, to see that there are no characters before. $ looks ahead, to see that there are no characters after. \b looks both ahead and behind, to see if the characters on either side differ in their 'word'-ness.

The lookahead and lookbehind assertions are generalizations of the anchor concept. Lookahead and lookbehind are zero-width assertions that let us specify which characters we want to test for. The lookahead assertion is denoted by (?=regexp) and the lookbehind assertion is denoted by (?<=fixed-regexp) . Some examples are

Note that the parentheses in (?=regexp) and (?<=regexp) are non-capturing, since these are zero-width assertions. Thus in the second regexp, the substrings captured are those of the whole regexp itself. Lookahead (?=regexp) can match arbitrary regexps, but lookbehind (?<=fixed-regexp) only works for regexps of fixed width, i.e., a fixed number of characters long. Thus (?<=(ab|bc)) is fine, but (?<=(ab)*) is not. The negated versions of the lookahead and lookbehind assertions are denoted by (?!regexp) and (?<!fixed-regexp) respectively. They evaluate true if the regexps do not match:

The \C is unsupported in lookbehind, because the already treacherous definition of \C would become even more so when going backwards.

Using independent subexpressions to prevent backtracking

The last few extended patterns in this tutorial are experimental as of 5.6.0. Play with them, use them in some code, but don't rely on them just yet for production code.

Independent subexpressions   are regular expressions, in the context of a larger regular expression, that function independently of the larger regular expression. That is, they consume as much or as little of the string as they wish without regard for the ability of the larger regexp to match. Independent subexpressions are represented by (?>regexp) . We can illustrate their behavior by first considering an ordinary regexp:

This obviously matches, but in the process of matching, the subexpression a* first grabbed the a . Doing so, however, wouldn't allow the whole regexp to match, so after backtracking, a* eventually gave back the a and matched the empty string. Here, what a* matched was dependent on what the rest of the regexp matched.

Contrast that with an independent subexpression:

The independent subexpression (?>a*) doesn't care about the rest of the regexp, so it sees an a and grabs it. Then the rest of the regexp ab cannot match. Because (?>a*) is independent, there is no backtracking and the independent subexpression does not give up its a . Thus the match of the regexp as a whole fails. A similar behavior occurs with completely independent regexps:

Here //g and \G create a 'tag team' handoff of the string from one regexp to the other. Regexps with an independent subexpression are much like this, with a handoff of the string to the independent subexpression, and a handoff of the string back to the enclosing regexp.

The ability of an independent subexpression to prevent backtracking can be quite useful. Suppose we want to match a non-empty string enclosed in parentheses up to two levels deep. Then the following regexp matches:

The regexp matches an open parenthesis, one or more copies of an alternation, and a close parenthesis. The alternation is two-way, with the first alternative [^()]+ matching a substring with no parentheses and the second alternative \([^()]*\) matching a substring delimited by parentheses. The problem with this regexp is that it is pathological: it has nested indeterminate quantifiers of the form (a+|b)+ . We discussed in Part 1 how nested quantifiers like this could take an exponentially long time to execute if there was no match possible. To prevent the exponential blowup, we need to prevent useless backtracking at some point. This can be done by enclosing the inner quantifier as an independent subexpression:

Here, (?>[^()]+) breaks the degeneracy of string partitioning by gobbling up as much of the string as possible and keeping it. Then match failures fail much more quickly.

Conditional expressions

A conditional expression   is a form of if-then-else statement that allows one to choose which patterns are to be matched, based on some condition. There are two types of conditional expression: (?(condition)yes-regexp) and (?(condition)yes-regexp|no-regexp) . (?(condition)yes-regexp) is like an 'if () {}'   statement in Perl. If the condition is true, the yes-regexp will be matched. If the condition is false, the yes-regexp will be skipped and perl will move onto the next regexp element. The second form is like an 'if () {} else {}'   statement in Perl. If the condition is true, the yes-regexp will be matched, otherwise the no-regexp will be matched.

The condition can have two forms. The first form is simply an integer in parentheses (integer) . It is true if the corresponding backreference \integer matched earlier in the regexp. The second form is a bare zero width assertion (?...) , either a lookahead, a lookbehind, or a code assertion (discussed in the next section).

The integer form of the condition allows us to choose, with more flexibility, what to match based on what matched earlier in the regexp. This searches for words of the form "$x$x" or "$x$y$y$x" :

The lookbehind condition allows, along with backreferences, an earlier part of the match to influence a later part of the match. For instance,

matches a DNA sequence such that it either ends in AAG , or some other base pair combination and C . Note that the form is (?(?<=AA)G|C) and not (?((?<=AA))G|C) ; for the lookahead, lookbehind or code assertions, the parentheses around the conditional are not needed.

A bit of magic: executing Perl code in a regular expression

Normally, regexps are a part of Perl expressions. Code evaluation   expressions turn that around by allowing arbitrary Perl code to be a part of a regexp. A code evaluation expression is denoted (?{code}) , with code a string of Perl statements.

Code expressions are zero-width assertions, and the value they return depends on their environment. There are two possibilities: either the code expression is used as a conditional in a conditional expression (?(condition)...) , or it is not. If the code expression is a conditional, the code is evaluated and the result (i.e., the result of the last statement) is used to determine truth or falsehood. If the code expression is not used as a conditional, the assertion always evaluates true and the result is put into the special variable $^R . The variable $^R can then be used in code expressions later in the regexp. Here are some silly examples:

Pay careful attention to the next example:

At first glance, you'd think that it shouldn't print, because obviously the ddd isn't going to match the target string. But look at this example:

Hmm. What happened here? If you've been following along, you know that the above pattern should be effectively the same as the last one -- enclosing the d in a character class isn't going to change what it matches. So why does the first not print while the second one does?

The answer lies in the optimizations the REx engine makes. In the first case, all the engine sees are plain old characters (aside from the ?{} construct). It's smart enough to realize that the string 'ddd' doesn't occur in our target string before actually running the pattern through. But in the second case, we've tricked it into thinking that our pattern is more complicated than it is. It takes a look, sees our character class, and decides that it will have to actually run the pattern to determine whether or not it matches, and in the process of running it hits the print statement before it discovers that we don't have a match.

To take a closer look at how the engine does optimizations, see the section Pragmas and debugging below.

More fun with ?{} :

The bit of magic mentioned in the section title occurs when the regexp backtracks in the process of searching for a match. If the regexp backtracks over a code expression and if the variables used within are localized using local , the changes in the variables produced by the code expression are undone! Thus, if we wanted to count how many times a character got matched inside a group, we could use, e.g.,

If we replace the  (?{local $c = $c + 1;})   with  (?{$c = $c + 1;})  , the variable changes are not undone during backtracking, and we get

Note that only localized variable changes are undone. Other side effects of code expression execution are permanent. Thus

The result $^R is automatically localized, so that it will behave properly in the presence of backtracking.

This example uses a code expression in a conditional to match the article 'the' in either English or German:

Note that the syntax here is (?(?{...})yes-regexp|no-regexp) , not (?((?{...}))yes-regexp|no-regexp) . In other words, in the case of a code expression, we don't need the extra parentheses around the conditional.

If you try to use code expressions with interpolating variables, perl may surprise you:

If a regexp has (1) code expressions and interpolating variables,or (2) a variable that interpolates a code expression, perl treats the regexp as an error. If the code expression is precompiled into a variable, however, interpolating is ok. The question is, why is this an error?

The reason is that variable interpolation and code expressions together pose a security risk. The combination is dangerous because many programmers who write search engines often take user input and plug it directly into a regexp:

If the $regexp variable contains a code expression, the user could then execute arbitrary Perl code. For instance, some joker could search for system('rm -rf *');   to erase your files. In this sense, the combination of interpolation and code expressions taints your regexp. So by default, using both interpolation and code expressions in the same regexp is not allowed. If you're not concerned about malicious users, it is possible to bypass this security check by invoking use re 'eval'  :

Another form of code expression is the pattern code expression  . The pattern code expression is like a regular code expression, except that the result of the code evaluation is treated as a regular expression and matched immediately. A simple example is

This final example contains both ordinary and pattern code expressions. It detects if a binary string 1101010010001... has a Fibonacci spacing 0,1,1,2,3,5,... of the 1 's:

Ha! Try that with your garden variety regexp package...

Note that the variables $s0 and $s1 are not substituted when the regexp is compiled, as happens for ordinary variables outside a code expression. Rather, the code expressions are evaluated when perl encounters them during the search for a match.

The regexp without the //x modifier is

and is a great start on an Obfuscated Perl entry :-) When working with code and conditional expressions, the extended form of regexps is almost necessary in creating and debugging regexps.

Pragmas and debugging

Speaking of debugging, there are several pragmas available to control and debug regexps in Perl. We have already encountered one pragma in the previous section, use re 'eval';  , that allows variable interpolation and code expressions to coexist in a regexp. The other pragmas are

The taint pragma causes any substrings from a match with a tainted variable to be tainted as well. This is not normally the case, as regexps are often used to extract the safe bits from a tainted variable. Use taint when you are not extracting safe bits, but are performing some other processing. Both taint and eval pragmas are lexically scoped, which means they are in effect only until the end of the block enclosing the pragmas.

The global debug and debugcolor pragmas allow one to get detailed debugging info about regexp compilation and execution. debugcolor is the same as debug, except the debugging information is displayed in color on terminals that can display termcap color sequences. Here is example output:

If you have gotten this far into the tutorial, you can probably guess what the different parts of the debugging output tell you. The first part

describes the compilation stage. STAR(4) means that there is a starred object, in this case 'a' , and if it matches, goto line 4, i.e., PLUS(7) . The middle lines describe some heuristics and optimizations performed before a match:

Then the match is executed and the remaining lines describe the process:

Each step is of the form n <x> <y>  , with <x> the part of the string matched and <y> the part not yet matched. The |  1:  STAR   says that perl is at line number 1 n the compilation list above. See Debugging regular expressions in perldebguts for much more detail.

An alternative method of debugging regexps is to embed print statements within the regexp. This provides a blow-by-blow account of the backtracking in an alternation:

Code expressions, conditional expressions, and independent expressions are experimental . Don't use them in production code. Yet.

This is just a tutorial. For the full story on perl regular expressions, see the perlre regular expressions reference page.

For more information on the matching m// and substitution s/// operators, see Regexp Quote-Like Operators in perlop . For information on the split operation, see split in perlfunc .

For an excellent all-around resource on the care and feeding of regular expressions, see the book Mastering Regular Expressions by Jeffrey Friedl (published by O'Reilly, ISBN 1556592-257-3).

AUTHOR AND COPYRIGHT

Copyright (c) 2000 Mark Kvale All rights reserved.

This document may be distributed under the same terms as Perl itself.

Acknowledgments

The inspiration for the stop codon DNA example came from the ZIP code example in chapter 7 of Mastering Regular Expressions .

The author would like to thank Jeff Pinyan, Andrew Johnson, Peter Haworth, Ronald J Kimball, and Joe Smith for all their helpful comments.

Regex – Perl, Assign to variable from regex match

parentheses perl regex syntax variable-assignment

I am relatively new to perl and there is an example snippet of code in check_ilo2_health.pl in which there is a piece of syntax that I do not understand how or why it works. The code snippet is parsing SSL client data, in this case XML, line by line.

An example of the XML in question:

So in this case the if statement takes the MESSAGE line of the XML sample. I understand that my ($msg) treats the variable as a sort of list and I understand how the regular expressions match; however, what I do not understand is the syntax such that $msg is assigned to No error . The perl seems to be playing around with parenthesis syntax and such for this to work. While it works I would like to know how it works. Any assistance would be appreciated.

Best Solution

See Perlretut, Extracting-matches :

... in scalar context, $time =~ /(\d\d):(\d\d):(\d\d)/ returns a true or false value. In list context, however, it returns the list of matched values ($1,$2,$3)

( $line =~ m/MESSAGE='(.*)'/) returns a list of the matches by the capturing groups. You have one capturing group, so the content of that is then stored in ($msg).

Related Solutions

Sql – insert into … values ( select … from … ).

This is standard ANSI SQL and should work on any DBMS

It definitely works for:

• MS SQL Server
• AWS RedShift
• Google Spanner

Regex – How to match all occurrences of a regex

Using scan should do the trick:

Related Question

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Mastering Perl by brian d foy

Get full access to Mastering Perl and 60K+ other titles, with a free 10-day trial of O'Reilly.

There are also live events, courses curated by job role, and more.

Chapter 2. Advanced Regular Expressions

Regular expressions, or just regexes, are at the core of Perl’s text processing, and certainly are one of the features that made Perl so popular. All Perl programmers pass through a stage where they try to program everything as regexes and, when that’s not challenging enough, everything as a single regex. Perl’s regexes have many more features than I can, or want, to present here, so I include those advanced features I find most useful and expect other Perl programmers to know about without referring to perlre , the documentation page for regexes.

References to Regular Expressions

I don’t have to know every pattern at the time that I code something. Perl allows me to interpolate variables into regexes. I might hard code those values, take them from user input, or get them in any other way I can get or create data. Here’s a tiny Perl program to do grep ’s job. It takes the firstF argument from the command line and uses it as the regex in the while statement. That’s nothing special (yet); we showed you how to do this in Learning Perl . I can use the string in $regex as my pattern, and Perl compiles it when it interpolates the string in the match operator: [ 1 ]

I can use this program from the command line to search for patterns in files. Here I search for the pattern new in all of the Perl programs in the current directory:

What happens if I give it an invalid regex? I try it with a pattern that has an opening parenthesis without its closing mate:

When I interpolate the regex in the match operator, Perl compiles the regex and immediately complains, stopping my program. To catch that, I want to compile the regex before I try to use it.

The qr// is a regex quoting operator that stores my regex in a scalar (and as a quoting operator, its documentation shows up in perlop ). The qr// compiles the pattern so it’s ready to use when I interpolate $regex in the match operator. I wrap the eval operator around the qr// to catch the error, even though I end up die -ing anyway:

The regex in $regex has all of the features of the match operator, including back references and memory variables. This pattern searches for a three-character sequence where the first and third characters are the same, and none of them are whitespace. The input is the plain text version of the perl documentation page, which I get with perldoc -t :

It’s a bit hard, at least for me, to see what Perl matched, so I can make another change to my grep program to see what matched. The $& variable holds the portion of the string that matched:

Now I see that my regex is matching a literal dot, character, literal dot, as in .8. :

Just for fun, how about seeing what matched in each memory group, the variables $1 , $2 , and so on? I could try printing their contents, whether or not I had capturing groups for them, but how many do I print? Perl already knows because it keeps track of all of that in the special arrays @- and @+ , which hold the string offsets for the beginning and end, respectively, for each match. That is, for the match string in $_ , the number of memory groups is the last index in @- or @+ (they’ll be the same length). The first element in each is for the part of the string matched (so, $& ), and the next element, with index 1 , is for $1 , and so on for the rest of the array. The value in $1 is the same as this call to substr :

To print the memory variables, I just have to go through the indices in the array @- :

Now I can see the part of the string that matched as well as the submatches:

If I change my pattern to have more submatches, I don’t have to change anything to see the additional matches:

(?imsx-imsx:PATTERN)

What if I want to do something a bit more complex for my grep program, such as a case-insensitive search? Using my program to search for either “Perl” or “perl” I have a couple of options, neither of which are too much work:

If I want to make the entire pattern case-insensitive, I have to do much more work, and I don’t like that. With the match operator, I could just add the /i flag on the end:

I could do that with the qr// operator, too, although this makes all patterns case-insensitive now:

To get around this, I can specify the match options inside my pattern. The special sequence (?imsx) allows me to turn on the features for the options I specify. If I want case-insensitivity, I can use (?i) inside the pattern. Case-insensitivity applies for the rest of the pattern after the (?i) (or for the rest of the enclosing parentheses):

In general, I can enable flags for part of a pattern by specifying which ones I want in the parentheses, possibly with the portion of the pattern they apply to, as shown in Table 2-1 .

Table 2-1. Options available in the (?options:PATTERN)

I can even group them:

If I preface the options with a minus sign, I turn off those features for that group:

This is especially useful since I’m getting my pattern from the command line. In fact, when I use the qr// operator to create my regex, I’m already using these. I’ll change my program to print the regex after I create it with qr// but before I use it:

When I print the regex, I see it starts with all of the options turned off. The string version of regex uses (?-OPTIONS:PATTERN) to turn off all of the options:

I can turn on case-insensitivity, although the string form looks a bit odd, turning off i just to turn it back on:

Perl’s regexes have many similar sequences that start with a parenthesis, and I’ll show a few of them as I go through this chapter. Each starts with an opening parenthesis followed by some characters to denote what’s going on. The full list is in perlre .

References As Arguments

Since references are scalars, I can use my compiled regex just like any other scalar, including storing it in an array or a hash, or passing it as the argument to a subroutine. The Test::More module, for instance, has a like function that takes a regex as its second argument. I can test a string against a regex and get richer output when it fails to match:

Since $string uses programmer instead of hacker , the test fails. The output shows me the string, what I expected, and the regex it tried to use:

The like function doesn’t have to do anything special to accept a regex as an argument, although it does check its reference type [ 2 ] before it tries to do its magic:

Since $regex is just a reference (of type Rexexp ), I can do reference sorts of things with it. I use isa to check the type, or get the type with ref :

Noncapturing Grouping, (?:PATTERN)

Parentheses in regexes don’t have to trigger memory. I can use them simply for grouping by using the special sequence (?:PATTERN) . This way, I don’t get unwanted data in my capturing groups.

Perhaps I want to match the names on either side of one of the conjunctions and or or . In @array I have some strings that express pairs. The conjunction may change, so in my regex I use the alternation and|or . My problem is precedence. The alternation is higher precedence than sequence, so I need to enclose the alternation in parentheses, (\S+) (and|or) (\S+) , to make it work:

The output shows me an unwanted consequence of grouping the alternation: the part of the string in the parentheses shows up in the memory variables as $2 ( Table 2-2 ). That’s an artifact.

Table 2-2. Unintended match memories

Using the parentheses solves my precedence problem, but now I have that extra memory variable. That gets in the way when I change the program to use a match in list context. All the memory variables, including the conjunction, show up in @names :

I want to simply group things without triggering memory. Instead of the regular parentheses I just used, I add ?: right after the opening parenthesis of the group, which turns them into noncapturing parentheses. Instead of (and|or) , I now have (?:and|or) . This form doesn’t trigger the memory variables, and they don’t count toward the numbering of the memory variables either. I can apply quantifiers just like the plain parentheses as well. Now I don’t get my extra element in @names :

Readable Regexes, /x and (?#...)

Regular expressions have a much deserved reputation of being hard to read. Regexes have their own terse language that uses as few characters as possible to represent virtually infinite numbers of possibilities, and that’s just counting the parts that most people use everyday.

Luckily for other people, Perl gives me the opportunity to make my regexes much easier to read. Given a little bit of formatting magic, not only will others be able to figure out what I’m trying to match, but a couple weeks later, so will I. We touched on this lightly in Learning Perl , but it’s such a good idea that I’m going to say more about it. It’s also in Perl Best Practices by Damian Conway (O’Reilly).

When I add the /x flag to either the match or substitution operators, Perl ignores literal whitespace in the pattern. This means that I spread out the parts of my pattern to make the pattern more discernible. Gisle Aas’s HTTP::Date module parses a date by trying several different regexes. Here’s one of his regular expressions, although I’ve modified it to appear on a single line, wrapped to fit on this page:

Quick: Can you tell which one of the many date formats that parses? Me neither. Luckily, Gisle uses the /x flag to break apart the regex and add comments to show me what each piece of the pattern does. With /x , Perl ignores literal whitespace and Perl-style comments inside the regex. Here’s Gisle’s actual code, which is much easier to understand:

Under /x , to match whitespace I have to specify it explicitly, either using \s , which matches any whitespace, any of \f\r\n\t , or their octal or hexadecimal sequences, such as \040 or \x20 for a literal space. [ 3 ] Likewise, if I need a literal hash symbol, # , I have to escape it too, \# .

I don’t have to use /x to put comments in my regex. The (?#COMMENT) sequence does that for me. It probably doesn’t make the regex any more readable at first glance, though. I can mark the parts of a string right next to the parts of the pattern that represent it. Just because you can use (?#) doesn’t mean you should. I think the patterns are much easier to read with /x :

Global Matching

In Learning Perl we told you about the /g flag that you can use to make all possible substitutions, but it’s more useful than that. I can use it with the match operator, where it does different things in scalar and list context. We told you that the match operator returns true if it matches and false otherwise. That’s still true (we wouldn’t have lied to you), but it’s not just a boolean value. The list context behavior is the most useful. With the /g flag, the match operator returns all of the memory matches:

Even though I only have one set of memory parentheses in my regular expression, it makes as many matches as it can. Once it makes a match, Perl starts where it left off and tries again. I’ll say more on that in a moment. I often run into another Perl idiom that’s closely related to this, in which I don’t want the actual matches, but just a count:

This uses a little-known but important rule: the result of a list assignment is the number of elements in the list on the right side. In this case, that’s the number of elements the match operator returns. This only works for a list assignment, which is assigning from a list on the right side to a list on the left side. That’s why I have the extra () in there.

In scalar context, the /g flag does some extra work we didn’t tell you about earlier. During a successful match, Perl remembers its position in the string, and when I match against that same string again, Perl starts where it left off in that string. It returns the result of one application of the pattern to the string:

When I match against that same string again, Perl gets the next match:

I can even look at the match position as I go along. The built-in pos() operator returns the match position for the string I give it (or $_ by default). Every string maintains its own position. The first position in the string is 0 , so pos() returns undef when it doesn’t find a match and has been reset, and this only works when I’m using the /g flag (since there’s no point in pos() otherwise):

When my match fails, Perl resets the value of pos() to undef . If I continue matching, I’ll start at the beginning (and potentially create an endless loop):

As a side note, I really hate these print statements where I use the concatenation operator to get the result of a function call into the output. Perl doesn’t have a dedicated way to interpolate function calls, so I can cheat a bit. I call the function in an anonymous array constructor, [ ... ] , and then immediately dereference it by wrapping @{ ... } around it: [ 4 ]

The pos() operator can also be an lvalue, which is the fancy programming way of saying that I can assign to it and change its value. I can fool the match operator into starting wherever I like. After I match the first word in $line , the match position is somewhere after the beginning of the string. After I do that, I use index to find the next h after the current match position. Once I have the offset for that h , I assign the offset to pos($line) so the next match starts from that position:

Global Match Anchors

So far, my subsequent matches can “float,” meaning they can start matching anywhere after the starting position. To anchor my next match exactly where I left off the last time, I use the \G anchor . It’s just like the beginning of string anchor, ^ , except for where \G anchors at the current match position. If my match fails, Perl resets pos() , and I start at the beginning of the string.

In this example, I anchor my pattern with \G . After that, I use noncapturing parentheses to group optional whitespace, \s* , and word match, \w+ . I use the /x flag to spread out the parts to enhance readability. My match only gets the first four words, since it can’t match the comma (it’s not in \w ) after the first hacker . Since the next match must start where I left off, which is the comma, and the only thing I can match is whitespace or word characters, I can’t continue. That next match fails, and Perl resets the match position to the beginning of $line :

I have a way to get around Perl resetting the match position. If I want to try a match without resetting the starting point even if it fails, I can add the /c flag, which simply means to not reset the match position on a failed match. I can try something without suffering a penalty. If that doesn’t work, I can try something else at the same match position. This feature is a poor man’s lexer. Here’s a simple-minded sentence parser:

Look at that example again. What if I wanted to add more things I could match? I’d have to add another branch to the decision structure. That’s no fun. That’s a lot of repeated code structure doing the same thing: match something, then return $1 and a description. It doesn’t have to be like that, though. I rewrite this code to remove the repeated structure. I can store the regexes in the @items array. I use the qr// quoter that I showed earlier, and I put the regexes in the order that I want to try them. The foreach loop goes through them successively until it finds one that matches. When it finds a match, it prints a message using the description and whatever showed up in $1 . If I want to add more tokens, I just add their description to @items :

Look at some of the things going on in this example. All matches need the /gc flags, so I add those flags to the match operator inside the foreach loop. My regex to match a word, however, also needs the /i flag. I can’t add that to the match operator because I might have other branches that don’t want it. I add the /i assertion to my word regex in @items , turning on case-insensitivity for just that regex. If I wanted to keep the nice formatting I had earlier, I could have made that (?ix) . As a side note, if most of my regexes should be case-insensitive, I could add /i to the match operator, then turn that off with (?-i) in the appropriate regexes.

Lookarounds

Lookarounds are arbitrary anchors for regexes. We showed several anchors in Learning Perl , such as ^ , $ , and \b , and I just showed the \G anchor. Using a lookaround, I can describe my own anchor as a regex, and just like the other anchors, they don’t count as part of the pattern or consume part of the string. They specify a condition that must be true, but they don’t add to the part of the string that the overall pattern matches.

Lookarounds come in two flavors: lookaheads that look ahead to assert a condition immediately after the current match position, and lookbehinds that look behind to assert a condition immediately before the current match position. This sounds simple, but it’s easy to misapply these rules. The trick is to remember that it anchors to the current match position and then figure out on which side it applies.

Both lookaheads and lookbehinds have two types: positive and negative . The positive lookaround asserts that its pattern has to match. The negative lookaround asserts that its pattern doesn’t match. No matter which I choose, I have to remember that they apply to the current match position, not anywhere else in the string.

Lookahead Assertions, (?=PATTERN) and (?!PATTERN)

Lookahead assertions let me peek at the string immediately ahead of the current match position. The assertion doesn’t consume part of the string, and if it succeeds, matching picks up right after the current match position.

Positive lookahead assertions

In Learning Perl , we included an exercise to check for both “Fred” and “Wilma” on the same line of input, no matter the order they appeared on the line. The trick we wanted to show to the novice Perler is that two regexes can be simpler than one. One way to do this repeats both Wilma and Fred in the alternation so I can try either order. A second try separates them into two regexes:

I can make a simple, single regex using a positive lookahead assertion , denoted by (?=PATTERN) . This assertion doesn’t consume text in the string, but if it fails, the entire regex fails. In this example, in the positive lookahead assertion I use .*Wilma . That pattern must be true immediately after the current match position:

Since I used that at the start of my pattern, that means it has to be true at the beginning of the string. Specifically, at the beginning of the string, I have to be able to match any number of characters except a newline followed by Wilma . If that succeeds, it anchors the rest of the pattern to its position (the start of the string). Figure 2-1 shows the two ways that can work, depending on the order of Fred and Wilma in the string. The .*Wilma anchors where it started matching. The elastic .* , which can match any number of non-newline characters, anchors at the start of the string.

Figure 2-1. The positive lookahead assertion (?=.*Wilma) anchors the pattern at the beginning of the string

It’s easier to understand lookarounds by seeing when they don’t work, though. I’ll change my pattern a bit by removing the .* from the lookahead assertion. At first it appears to work, but it fails when I reverse the order of Fred and Wilma in the string:

Figure 2-2 shows what happens. In the first case, the lookahead anchors at the start of Wilma . The regex tried the assertion at the start of the string, found that it didn’t work, then moved over a position and tried again. It kept doing this until it got to Wilma . When it succeeded it set the anchor. Once it sets the anchor, the rest of the pattern has to start from that position.

Figure 2-2. The positive lookahead assertion (?=Wilma) anchors the pattern at Wilma

In the first case, .*Fred can match from that anchor because Fred comes after Wilma . The second case in Figure 2-2 does the same thing. The regex tries that assertion at the beginning of the string, finds that it doesn’t work, and moves on to the next position. By the time the lookahead assertion matches, it has already passed Fred . The rest of the pattern has to start from the anchor, but it can’t match.

Since the lookahead assertions don’t consume any of the string, I can use it in a pattern for split when I don’t really want to discard the parts of the pattern that match. In this example, I want to break apart the words in the studly cap string. I want to split it based on the initial capital letter. I want to keep the initial letter, though, so I use a lookahead assertion instead of a character-consuming string. This is different from the separator retention mode because the split pattern isn’t really a separator; it’s just an anchor:

Negative lookahead assertions

Suppose I want to find the input lines that contain Perl , but only if that isn’t Perl6 or Perl 6 . I might try a negated character class to specify the pattern right after the l in Perl to ensure that the next character isn’t a 6 . I also use the word boundary anchors \b because I don’t want to match in the middle of other words, such as “BioPerl” or “PerlPoint”:

I’ll try this with some sample input:

It doesn’t work for all the lines it should. It only finds four of the lines that have Perl without a trailing 6 , and a line that has a space between Perl and 6 :

That doesn’t work because there has to be a character after the l in Perl . Not only that, I specified a word boundary. If that character after the l is a nonword character, such as the " in I just say "Perl" , the word boundary at the end fails. If I take off the trailing \b , now PerlPoint matches. I haven’t even tried handling the case where there is a space between Perl and 6 . For that I’ll need something much better.

To make this really easy, I can use a negative lookahead assertion. I don’t want to match a character after the l , and since an assertion doesn’t match characters, it’s the right tool to use. I just want to say that if there’s anything after Perl , it can’t be a 6 , even if there is some whitespace between them. The negative lookahead assertion uses (?!PATTERN) . To solve this problem, I use \s?6 as my pattern, denoting the optional whitespace followed by a 6 :

Now the output finds all of the right lines:

Remember that (?!PATTERN) is a lookahead assertion, so it looks after the current match position. That’s why this next pattern still matches. The lookahead asserts that right before the b in bar that the next thing isn’t foo . Since the next thing is bar , which is not foo , it matches. People often confuse this to mean that the thing before bar can’t be foo , but each uses the same starting match position, and since bar is not foo , they both work:

Lookbehind Assertions, (?<!PATTERN) and (?<=PATTERN)

Instead of looking ahead at the part of the string coming up, I can use a lookbehind to check the part of the string the regular expression engine has already processed. Due to Perl’s implementation details, the lookbehind assertions have to be a fixed width, so I can’t use variable width quantifiers in them.

Now I can try to match bar that doesn’t follow a foo . In the previous section I couldn’t use a negative lookahead assertion because that looks forward in the string. A negative lookbehind, denoted by (?<!PATTERN) , looks backward. That’s just what I need. Now I get the right answer:

Now, since the regex has already processed that part of the string by the time it gets to bar , my lookbehind assertion can’t be a variable width pattern. I can’t use the quantifiers to make a variable width pattern because the engine is not going to backtrack in the string to make the lookbehind work. I won’t be able to check for a variable number of o s in fooo :

When I try that, I get the error telling me that I can’t do that, and even though it merely says not implemented , don’t hold your breath waiting for it:

The positive lookbehind assertion also looks backward, but its pattern must not match. The only time I seem to use these are in substitutions in concert with another assertion. Using both a lookbehind and a lookahead assertion, I can make some of my substitutions easier to read.

For instance, throughout the book I’ve used variations of hyphenated words because I couldn’t decide which one I should use. Should it be builtin or built-in ? Depending on my mood or typing skills, I used either of them. [ 5 ]

I needed to clean up my inconsistency. I knew the part of the word on the left of the hyphen, and I knew the text on the right of the hyphen. At the position where they meet, there should be a hyphen. If I think about that for a moment, I’ve just described the ideal situation for lookarounds: I want to put something at a particular position, and I know what should be around it. Here’s a sample program to use a positive lookbehind to check the text on the left and a positive lookahead to check the text on the right. Since the regex only matches when those sides meet, that means that it’s discovered a missing hyphen. When I make the substitution, it put the hyphen at the match position, and I don’t have to worry about the particular text:

If that’s not a complicated enough example, try this one. Let’s use the lookarounds to add commas to numbers. Jeffery Friedl shows one attempt in Mastering Regular Expressions , adding commas to the U.S. population: [ 6 ]

That works, mostly. The positive lookbehind (?<=\d) wants to match a number, and the positive lookahead (?=(?:\d\d\d)+$) wants to find groups of three digits all the way to the end of the string. This breaks when I have floating point numbers, such as currency. For instance, my broker tracks my stock positions to four decimal places. When I try that substitution, I get no comma on the left side of the decimal point and one of the fractional side. It’s because of that end of string anchor:

I can modify that a bit. Instead of the end of string anchor, I’ll use a word boundary, \b . That might seem weird, but remember that a digit is a word character. That gets me the comma on the left side, but I still have that extra comma:

What I really want for that first part of the regex is to use the lookbehind to match a digit, but not when it’s preceded by a decimal point. That’s the description of a negative lookbehind, (?<!\.\d) . Since all of these match at the same position, it doesn’t matter that some of them might overlap as long as they all do what I need:

That works! It’s a bit too bad that it does because I’d really like an excuse to get a negative lookahead in there. It’s too complicated already, so I’ll just add the /x to practice what I preach:

Deciphering Regular Expressions

While trying to figure out a regex, whether one I found in someone else’s code or one I wrote myself (maybe a long time ago), I can turn on Perl’s regex debugging mode. [ 7 ] Perl’s -D switch turns on debugging options for the Perl interpreter (not for your program, as in Chapter 4 ). The switch takes a series of letters or numbers to indicate what it should turn on. The -Dr option turns on regex parsing and execution debugging.

I can use a short program to examine a regex. The first argument is the match string and the second argument is the regular expression. I save this program as explain-regex :

When I try this with the target string Just another Perl hacker, and the regex Just another (\S+) hacker, , I see two major sections of output, which the perldebguts documentation explains at length. First, Perl compiles the regex, and the -Dr output shows how Perl parsed the regex. It shows the regex nodes, such as EXACT and NSPACE , as well as any optimizations, such as anchored "Just another " . Second, it tries to match the target string, and shows its progress through the nodes. It’s a lot of information, but it shows me exactly what it’s doing:

The re pragma, which comes with Perl, has a debugging mode that doesn’t require a -DDEBUGGING enabled interpreter. Once I turn on use re 'debug' , it applies to the entire program. It’s not lexically scoped like most pragmata. I modify my previous program to use the re pragma instead of the command-line switch:

I don’t have to modify my program to use re since I can also load it from the command line:

When I run this program with a regex as its argument, I get almost the same exact output as my previous -Dr example.

The YAPE::Regex::Explain , although a bit old, might be useful in explaining a regex in mostly plain English. It parses a regex and provides a description of what each part does. It can’t explain the semantic purpose, but I can’t have everything. With a short program I can explain the regex I specify on the command line:

When I run the program even with a short, simple regex, I get plenty of output:

Final Thoughts

It’s almost the end of the chapter, but there are still so many regular expression features I find useful. Consider this section a quick tour of the things you can look into on your own.

I don’t have to be content with the simple character classes such as \w (word characters), \d (digits), and the others denoted by slash sequences. I can also use the POSIX character classes. I enclose those in the square brackets with colons on both sides of the name:

I negate those with a caret, ^ , after the first colon:

I can say the same thing in another way by specifying a named property. The \p{Name} sequence (little p) includes the characters for the named property, and the \P{Name} sequence (big P) is its complement:

The Regexp::Common module provides pretested and known-to-work regexes for, well, common things such as web addresses, numbers, postal codes, and even profanity. It gives me a multilevel hash %RE that has as its values regexes. If I don’t like that, I can use its function interface:

If I want to build up my own pattern, I can use Regexp::English , which uses a series of chained methods to return an object that stands in for a regex. It’s probably not something you want in a real program, but it’s fun to think about:

If you really want to get into the nuts and bolts of regular expressions, check out O’Reilly’s Mastering Regular Expressions by Jeffrey Friedl. You’ll not only learn some advanced features, but how regular expressions work and how you can make yours better.

This chapter covered some of the more useful advanced features of Perl’s regex engine. The qr() quoting operator lets me compile a regex for later and gives it back to me as a reference. With the special (?) sequences, I can make my regular expression much more powerful, as well as less complicated. The \G anchor allows me to anchor the next match where the last one left off, and using the /c flag, I can try several possibilities without resetting the match position if one of them fails.

Further Reading

perlre is the documentation for Perl regexes, and perlretut gives a regex tutorial. Don’t confuse that with perlreftut , the tutorial on references. To make it even more complicated, perlreref is the regex quick reference.

The details for regex debugging shows up in perldebguts . It explains the output of -Dr and re 'debug' .

Perl Best Practices has a section on regexes, and gives the \x “Extended Formatting” pride of place.

Mastering Regular Expressions covers regexes in general, and compares their implementation in different languages. Jeffrey Friedl has an especially nice description of lookahead and lookbehind operators. If you really want to know about regexes, this is the book to get.

Simon Cozens explains advanced regex features in two articles for Perl.com: “Regexp Power” ( http://www.perl.com/pub/a/2003/06/06/regexps.html ) and “Power Regexps, Part II” ( http://www.perl.com/pub/a/2003/07/01/regexps.html ).

The web site http://www.regular-expressions.info has good discussions about regular expressions and their implementations in different languages.

[ 1 ] As of Perl 5.6, if the string does not change, Perl will not recompile that regex. Before Perl 5.6, I had to use the /o flag to get that behavior. I can still use /o if I don’t want to recompile the pattern even if the variable changes.

[ 2 ] That actually happens in the maybe_regex method in Test::Builder .

[ 3 ] I can also escape a literal space character with a \ , but since I can’t really see the space, I prefer to use something I can see, such as \x20 .

[ 4 ] This is the same trick I need to use to interpolate function calls inside a string: print "Result is: @{ [ func(@args) ] }" .

[ 5 ] As a publisher, O’Reilly Media has dealt with this many times, so it maintains a word list to say how they do it, although that doesn’t mean that authors like me read it: http://www.oreilly.com/oreilly/author/stylesheet.html .

[ 6 ] The U.S. Census Bureau has a population clock so you can use the latest number if you’re reading this book a long time from now: http://www.census.gov/main/www/popclock.html .

[ 7 ] The regular expression debugging mode requires an interpreter compiled with -DDEBUGGING . Running perl -V shows the interpreter’s compilation options.

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TABLE OF CONTENTS (HIDE)

Perl tutorial, regular expressions, file io & text processing.

Perl is famous for processing text files via regular expressions.

Regular Expressions in Perl

A Regular Expression (or Regex ) is a pattern (or filter ) that describes a set of strings that matches the pattern.  In other words, a regex accepts a certain set of strings and rejects the rest.

I shall assume that you are familiar with Regex syntax. Otherwise, you could read:

• " Regex Syntax Summary " for a summary of regex syntax and examples.
• " Regular Expressions " for full coverage.

Perl makes extensive use of regular expressions with many built-in syntaxes and operators. In Perl (and JavaScript), a regex is delimited by a pair of forward slashes (default), in the form of / regex / . You can use built-in operators:

• m/ regex / modifier : Match against the regex .
• s/ regex / replacement / modifier : Substitute matched substring(s) by the replacement.

Matching Operator m//

You can use matching operator m// to check if a regex pattern exists in a string. The syntax is:

Instead of using forward-slashes ( / ) as delimiter, you could use other non-alphanumeric characters such as ! , @ and % in the form of m! regex ! modifiers m@ regex @ modifiers or m% regex % modifiers . However, if forward-slash ( / ) is used as the delimiter, the operator m can be omitted in the form of / regex / modifiers . Changing the default delimiter is confusing, and not recommended.

m// , by default, operates on the default variable $_ . It returns true if $_ matches regex; and false otherwise.

Example 1: Regex [0-9]+

Example 2: extracting the matched substrings.

The built-in array variables @- and @+ keep the start and end positions of the matched substring, where $-[0] and $+[0] for the full match, and $-[n] and $+[n] for back references $1 , $2 , ..., $n , ....

Example 3: Modifier 'g' (global)

By default, m// finds only the first match. To find all matches, include 'g' (global) modifier.

Operators =~ and !~

By default, the matching operators operate on the default variable $_ . To operate on other variable instead of $_ , you could use the =~ and !~ operators as follows:

When used with m// , =~ behaves like comparison ( == or eq ).

Example 4: =~ Operator

Substitution operator s///.

You can substitute a string (or a portion of a string) with another string using s/// substitution operator. The syntax is:

Similar to m// , s/// operates on the default variable $_ by default. To operate on other variable, you could use the =~ and !~ operators. When used with s/// , =~ behaves like assignment ( = ).

Example 5: s///

Modifiers (such as /g , /i , /e , /o , /s and /x ) can be used to control the behavior of m// and s/// .

• g (global): By default, only the first occurrence of the matching string of each line is processed. You can use modifier /g to specify global operation.
• i (case-insensitive): By default, matching is case-sensitive. You can use the modifier /i to enable case in-sensitive matching.
• m (multiline): multiline string, affecting position anchor ^ , $ , \A , \Z .
• s : permits metacharacter . (dot) to match the newline.

Parenthesized Back-References & Matched Variables $1 , ..., $9

Parentheses ( ) serve two purposes in regex:

• Firstly, parentheses ( ) can be used to group sub-expressions for overriding the precedence or applying a repetition operator. For example, /(a|e|i|o|u){3,5}/ is the same as /a{3,5}|e{3,5}|i{3,5}|o{3,5}|u{3,5}/ .
• Secondly, parentheses are used to provide the so called back-references . A back-reference contains the matched sub-string. For examples, the regex /(\S+)/ creates one back-reference (\S+) , which contains the first word (consecutive non-spaces) in the input string; the regex /(\S+)\s+(\S+)/ creates two back-references: (\S+) and another (\S+) , containing the first two words, separated by one or more spaces \s+ .

The back-references are stored in special variables $1 , $2 , …, $9 , where $1 contains the substring matched the first pair of parentheses, and so on. For example, /(\S+)\s+(\S+)/ creates two back-references which matched with the first two words. The matched words are stored in $1 and $2 , respectively.

For example, the following expression swap the first and second words:

Back-references can also be referenced in your program.

For example,

The parentheses creates one back-reference, which matches the first word of the $str if there is one, and is placed inside the scalar variable $word . If there is no match, $word is UNDEF .

Another example,

The 2 pairs of parentheses place the first two words (separated by one or more white-spaces) of the $str into variables $word1 and $word2 if there are more than two words; otherwise, both $word1 and $word2 are UNDEF . Note that regular expression matching must be complete and there is no partial matching.

\1 , \2 , \3 has the same meaning as $1 , $2 , $3 , but are valid only inside the s/// or m// . For example, /(\S+)\s\1/ matches a pair of repeated words, separated by a white-space.

Character Translation Operator tr///

You can use translator operator to translate a character into another character. The syntax is:

replaces or translates fromchars to tochars in $_ , and returns the number of characters replaced.

For examples,

Instead of forward slash ( / ), you can use parentheses () , brackets [] , curly bracket {} as delimiter, e.g.,

If tochars is shorter than fromchars , the last character of tochars is used repeatedly.

tr/// returns the number of replaced characters. You can use it to count the occurrence of certain characters. For examples,

Modifiers /c , /d and /s for tr///

• /c : complements (inverses) fromchars .
• /d : deletes any matched but un-replaced characters.
• /s : squashes duplicate characters into just one.

String Functions: split and join

split( regex , str , [ numItems ]) : Splits the given str using the regex , and return the items in an array. The optional third parameter specifies the maximum items to be processed.

join( joinStr , strList ) : Joins the items in strList with the given joinStr (possibly empty).

Functions grep , map

• grep( regex , array ) : selects those elements of the array , that matches regex .
• map( regex , array ) : returns a new array constructed by applying regex to each element of the array .

File Input/Output

Filehandles are data structure which your program can use to manipulate files.  A filehandle acts as a gate between your program and the files , directories , or other programs .  Your program first opens a gate, then sends or receives data through the gate, and finally closes the gate. There are many types of gates: one-way vs. two-way, slow vs. fast, wide vs. narrow.

Naming Convention : use uppercase for the name of the filehandle, e.g., FILE , DIR , FILEIN , FILEOUT , and etc.

Once a filehandle is created and connected to a file (or a directory, or a program), you can read or write to the underlying file through the filehandle using angle brackets, e.g., < FILEHANDLE > .

Example : Read and print the content of a text file via a filehandle.

Example : Search and print lines containing a particular search word.

Example : Print the content of a directory via a directory handle.

You can use C-style's printf for formatted output to file.

File Handling Functions

Function open : open( filehandle , string ) opens the filename given by string and associates it with the filehandle . It returns true if success and UNDEF otherwise.

• If string begins with < (or nothing), it is opened for reading.
• If string begins with > , it is opened for writing.
• If string begins with >> , it is opened for appending.
• If string begins with +< , +> , +>> , it is opened for both reading and writing.
• If string is - , STDIN is opened.
• If string is >- , STDOUT is opened.
• If string begins with -| or |- , your process will fork() to execute the pipe command .

Function close : close( filehandle ) closes the file associated with the filehandle . When the program exits, Perl closes all opened filehandles. Closing of file flushes the output buffer to the file.  You only have to explicitly close the file in case the user aborts the program, to ensure data integrity.

A common procedure for modifying a file is to:

• Read in the entire file with open(FILE, $filename) and @lines = <FILE> .
• Close the filehandle.
• Operate upon @lines (which is in the fast RAM) rather than FILE (which is in the slow disk).
• Write the new file contents using open(FILE, “>$filename”) and print FILE @lines .
• Close the file handle.

Example : Read the contents of the entire file into memory; modify and write back to disk.

Example : Reading from a file

Example : Writing to a file

Example : Appending to a file

In-Place Editing

Instead of reading in one file and write to another file, you could do in-place editing by specifying –i flag or use the special variable $^I .

• The –i backupExtension flag tells Perl to edit files in-place.  If a backupExtension is provided, a backup file will be created with the backupExtension .
• The special variable $^I= backupExtension does the same thing.

Example : In-place editing using –i flag

Example :  In-place editing using $^I special variable.

Functions seek , tell , truncate

seek( filehandle , position , whence ) : moves the file pointer of the filehandle to position , as measured from whence .  seek() returns 1 upon success and 0 otherwise.  File position is measured in bytes.  whence of 0 measured from the beginning of the file; 1 measured from the current position; and 2 measured from the end.  For example:

tell( filehandle ) : returns the current file position of filehandle .

truncate( FILE , length ) : truncates FILE to length bytes.  FILE can be either a filehandle or a file name.

To find the length of a file, you could:

Example : Truncate the last 2 bytes if they begin with \x0D ,

Function eof

eof( filehandle ) returns 1 if the file pointer is positioned at the end of the file or if the filehandle is not opened.

Reading Bytes Instead of Lines

The function read( filehandle , var , length , offset ) reads length bytes from filehandle starting from the current file pointer, and saves into variable var starting from offset (if omitted, default is 0).  The bytes includes \x0A , \x0D etc.

Piping Data To and From a Process

If you wish your program to receive data from a process or want your program to send data to a process, you could open a pipe to an external program.

• open( handle , " command |") lets you read from the output of command .
• open( handle , "| command ") lets you write to the input of command .

Both of these statements return the Process ID (PID) of the command .

Example : The dir command lists the current directory.  By opening a pipe from dir , you can access its output.

Example : This example shows how you can pipe input into the sendmail program.

You cannot pipe data both to and from a command .  If you want to read the output of a command that you have opened with the | command , send the output to a file.  For example,

Deleting file: Function unlink

unlink( FILES ) deletes the FILES , returning the number of files deleted.  Do not use unlink() to delete a directory, use rmdir() instead. For example,

Inspecting Files

You can inspect a file using (- test FILE ) condition.  The condition returns true if FILE satisfies test .  FILE can be a filehandle or filename.  The available test are:

• -e : exists.
• -f : plain file.
• -d : directory.
• -T : seems to be a text file (data from 0 to 127).
• -B : seems to be a binary file (data from 0 to 255).
• -r : readable.
• -w : writable.
• -x : executable.
• -s : returns the size of the file in bytes.
• -z : empty (zero byte).

Function stat and lsstat

The function stat( FILE ) returns a 13-element array giving the vital statistics of FILE .  lsstat( SYMLINK ) returns the same thing for the symbolic link SYMLINK .

The elements are:

For example: The command

prints the file size of " test.txt ".

Accessing the Directories

• opendir( DIRHANDLE , dirname ) opens the directory dirname .
• closedir( DIRHANDLE ) closes the directory handle.
• readdir( DIRHANDLE ) returns the next file from DIRHANDLE in a scalar context, or the rest of the files in the array context.
• glob( string ) returns an array of filenames matching the wildcard in string ,  e.g., glob('*.dat') and glob('test??.txt') .
• mkdir( dirname , mode ) creates the directory dirname with the protection specified by mode .
• rmdir(dirname) deletes the directory dirname , only if it is empty.
• chdir( dirname ) changes the working directory to dirname .
• chroot( dirname ) makes dirname the root directory "/" for the current process, used by superuser only.

Example : Print the contents of a given directory.

Example :  Removing empty files in a given directory

Example : Display files matches " *.txt "

Example : Display files matches the command-line pattern.

Standard Filehandles

Perl defines the following standard filehandles:

• STDIN – Standard Input, usually refers to the keyboard.
• STDOUT – Standard Output, usually refers to the console.
• STDERR – Standard Error, usually refers to the console.
• ARGV – Command-line arguments.

For example:

When you use an empty angle brackets <> to get inputs from user, it uses the STDIN filehandle; when you get the inputs from the command-line, it uses ARGV filehandle.  Perl fills in STDIN or ARGV for you automatically.  Whenever you use print() function, it uses the STDOUT filehandler.

<> behaves like <ARGV> when there is still data to be read from the command-line files, and behave like <STDIN> otherwise.

Text Formatting

Function write.

write( filehandle ) : printed formatted text to filehandle , using the format associated with filehandle . If filehandle is omitted, STDOUT would be used.

Declaring format

Picture field @< , @| , @>.

• @< : left-flushes the string on the next line of formatting texts.
• @> : right-flushes the string on the next line of formatting texts.
• @| : centers the string on the next line of the formatting texts.

@< , @> , @| can be repeated to control the number of characters to be formatted. The number of characters to be formatted is same as the length of the picture field. @###.## formats numbers by lining up the decimal points under " . ".

Printing Formatting String printf

printf( filehandle , template , array ) : prints a formatted string to filehandle (similar to C's fprintf() ). For example,

The available formatting fields are:

REFERENCES & RESOURCES

Latest version tested: ??? Last modified: December, 2012

• Regex to match perl variable

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You need a $ at the end, otherwise it's just matches as far as it can and ignores the rest. So it should be:

I needed to solve this problem to create a simple source code analyzer. This subroutine extracts Perl user variables from an input section of code

Test it with this code:

my @variables = extractVars('$a $a{b} $a[c] $scalar @list %hash $list[0][1] $list[-1] $hash{foo}{bar} $aref->{foo} $href->{foo}->{bar} @$aref %$hash_ref %{$aref->{foo}} $hash{\'foo\'} "$a" "$var{abc}"');

It does NOT work if the variable name contains spaces, for example:

• $hash{"baz qux"}
• ${ $var->{foo} }[0]

Parsing Perl is difficult, and the rules for what is and is not a variable are complicated. If you're attempting to parse Perl, consider using PPI instead. It can parse a Perl program and do things like find all the variables. PPI is what perlcritic uses to do its job.

If you want to try and do it anyway, here's some edge cases to consider...

And of course the other sigils @%* . And detecting if something is inside a single quoted string. Which is my way of strongly encouraging you to use PPI rather than try to do it yourself.

If you want practice, realistic practice is to pull the variable out of a larger string, rather than do exact matches.

This uses the () capture operator to grab just the variable. It also uses the /x modifier to make the regex easier to read and alternative delimiters to avoid leaning toothpick syndrome . Using \w instead of A-Z ensures that Unicode characters will be picked up when utf8 is on, and that they won't when its off. Finally, qr is used to build up the regex in pieces. Filling in the gaps is left as an exercise.

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