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

 "Hello World" =~ /World/;  # matches

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 truevalue if the regex matched, or false if the regex did not match. Inour case, World matches the second word in "Hello World", so theexpression is true. This idea has several variations.

Expressions like this are useful in conditionals:

 print "It matches\n" if "Hello World" =~ /World/;

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

 print "It doesn't match\n" if "Hello World" !~ /World/;

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

 $greeting = "World";
  print "It matches\n" if "Hello World" =~ /$greeting/;

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

 $_ = "Hello World";
  print "It matches\n" if /World/;

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

 "Hello World" =~ m!World!;   # matches, delimited by '!'
  "Hello World" =~ m{World};   # matches, note the matching '{}'
  "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
  # '/' becomes an ordinary char

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

 "Hello World" =~ /world/;  # doesn't match, case sensitive
  "Hello World" =~ /o W/; # matches, ' ' is an ordinary char
  "Hello World" =~ /World /; # doesn't match, no ' ' at end

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

 "Hello World" =~ /o/;   # matches 'o' in 'Hello'
  "That hat is red" =~ /hat/; # matches 'hat' in 'That'

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

 {}[]()^$.|*+?\

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

 "2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
  "2+2=4" =~ /2\+2/;   # matches, \+ is treated like an ordinary +
  'C:\WIN32' =~ /C:\\WIN/;   # matches
  "/usr/bin/perl" =~ /\/usr\/bin\/perl/;  # matches

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

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

 "1000\t2000" =~ m(0\t2)  # matches
  "cat"  =~ /\143\x61\x74/ # matches in ASCII, but a weird way to spell cat

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

 $foo = 'house';
  'cathouse' =~ /cat$foo/;   # matches
  'housecat' =~ /${foo}cat/; # matches

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

 "housekeeper" =~ /keeper/; # matches
  "housekeeper" =~ /^keeper/; # doesn't match
  "housekeeper" =~ /keeper$/; # matches
  "housekeeper\n" =~ /keeper$/;  # matches
  "housekeeper" =~ /^housekeeper$/;  # matches

Using character classes

A character class allows a set of possible characters, rather thanjust a single character, to match at a particular point in a regex.Character classes are denoted by brackets [...], with the set ofcharacters to be possibly matched inside. Here are some examples:

 /cat/; # matches 'cat'
  /[bcr]at/; # matches 'bat', 'cat', or 'rat'
  "abc" =~ /[cab]/; # matches 'a'

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

 /[yY][eE][sS]/; # match 'yes' in a case-insensitive way
  # 'yes', 'Yes', 'YES', etc.
  /yes/i; # also match 'yes' in a case-insensitive way

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

Character classes also have ordinary and special characters, but thesets of ordinary and special characters inside a character class aredifferent than those outside a character class. The specialcharacters for a character class are -]\^$ and are matched using anescape:

   /[\]c]def/; # matches ']def' or 'cdef'
    $x = 'bcr';
    /[$x]at/;   # matches 'bat, 'cat', or 'rat'
    /[\$x]at/;  # matches '$at' or 'xat'
    /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'

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

 /item[0-9]/;  # matches 'item0' or ... or 'item9'
  /[0-9a-fA-F]/;  # matches a hexadecimal digit

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

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

 /[^a]at/;  # doesn't match 'aat' or 'at', but matches
    # all other 'bat', 'cat, '0at', '%at', etc.
  /[^0-9]/;  # matches a non-numeric character
  /[a^]at/;  # matches 'aat' or '^at' here '^' is ordinary

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

\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 outsideof character classes. Here are some in use:

 /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
  /[\d\s]/; # matches any digit or whitespace character
  /\w\W\w/; # matches a word char, followed by a
   # non-word char, followed by a word char
  /..rt/;   # matches any two chars, followed by 'rt'
  /end\./;  # matches 'end.'
  /end[.]/; # same thing, matches 'end.'

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

 $x = "Housecat catenates house and cat";
  $x =~ /\bcat/;  # matches cat in 'catenates'
  $x =~ /cat\b/;  # matches cat in 'housecat'
  $x =~ /\bcat\b/;  # matches 'cat' at end of string

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

Matching this or that

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

 "cats and dogs" =~ /cat|dog|bird/;  # matches "cat"
  "cats and dogs" =~ /dog|cat|bird/;  # matches "cat"

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

 "cats"  =~ /c|ca|cat|cats/; # matches "c"
  "cats"  =~ /cats|cat|ca|c/; # matches "cats"

At a given character position, the first alternative that allows theregex match to succeed will be the one that matches. Here, all thealternatives 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 betreated as a single unit. Parts of a regex are grouped by enclosingthem in parentheses. The regex house(cat|keeper) means matchhouse followed by either cat or keeper. Some more examplesare

 /(a|b)b/; # matches 'ab' or 'bb'
  /(^a|b)c/;   # matches 'ac' at start of string or 'bc' anywhere
 
  /house(cat|)/;  # matches either 'housecat' or 'house'
  /house(cat(s|)|)/;  # matches either 'housecats' or 'housecat' or
  # 'house'.  Note groups can be nested.
 
  "20" =~ /(19|20|)\d\d/;  # matches the null alternative '()\d\d',
  # because '20\d\d' can't match

Extracting matches

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

 # extract hours, minutes, seconds
  $time =~ /(\d\d):(\d\d):(\d\d)/;  # match hh:mm:ss format
  $hours = $1;
  $minutes = $2;
  $seconds = $3;

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

 ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);

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

 /(ab(cd|ef)((gi)|j))/;
  1  2  34

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

 /(\w\w\w)\s\g1/; # find sequences like 'the the' in string

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

Matching repetitions

The quantifier metacharacters ?, *, +, and {} allow usto determine the number of repeats of a portion of a regex weconsider to be a match. Quantifiers are put immediately after thecharacter, character class, or grouping that we want to specify. Theyhave 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 mtimes.
a{n,} = match at least n or more times
a{n} = match exactly n times

Here are some examples:

 /[a-z]+\s+\d*/;  # match a lowercase word, at least some space, and
  # any number of digits
  /(\w+)\s+\g1/; # match doubled words of arbitrary length
  $year =~ /^\d{2,4}$/;  # make sure year is at least 2 but not more
    # than 4 digits
  $year =~ /^\d{4}$|^\d{2}$/; # better match; throw out 3 digit dates

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

 $x = 'the cat in the hat';
  $x =~ /^(.*)(at)(.*)$/; # matches,
  # $1 = 'the cat in the h'
  # $2 = 'at'
  # $3 = ''   (0 matches)

The first quantifier .* grabs as much of the string as possiblewhile still having the regex match. The second quantifier .* hasno string left to it, so it matches 0 times.

More matching

There are a few more things you might want to know about matchingoperators.The global modifier //g allows the matching operator to matchwithin a string as many times as possible. In scalar context,successive matches against a string will have //g jump from matchto 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,

 $x = "cat dog house"; # 3 words
  while ($x =~ /(\w+)/g) {
  print "Word is $1, ends at position ", pos $x, "\n";
  }

prints

 Word is cat, ends at position 3
  Word is dog, ends at position 7
  Word is house, ends at position 13

A failed match or changing the target string resets the position. Ifyou 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 ifthere are no groupings, a list of matches to the whole regex. So

 @words = ($x =~ /(\w+)/g);  # matches,
  # $word[0] = 'cat'
  # $word[1] = 'dog'
  # $word[2] = 'house'

Search and replace

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

 $x = "Time to feed the cat!";
  $x =~ s/cat/hacker/;   # $x contains "Time to feed the hacker!"
  $y = "'quoted words'";
  $y =~ s/^'(.*)'$/$1/;  # strip single quotes,
    # $y contains "quoted words"

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

 $x = "I batted 4 for 4";
  $x =~ s/4/four/;   # $x contains "I batted four for 4"
  $x = "I batted 4 for 4";
  $x =~ s/4/four/g;  # $x contains "I batted four for four"

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

 $x = "I like dogs.";
  $y = $x =~ s/dogs/cats/r;
  print "$x $y\n"; # prints "I like dogs. I like cats."
 
  $x = "Cats are great.";
  print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~ s/Frogs/Hedgehogs/r, "\n";
  # prints "Hedgehogs are great."
 
  @foo = map { s/[a-z]/X/r } qw(a b c 1 2 3);
  # @foo is now qw(X X X 1 2 3)

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

 # reverse all the words in a string
  $x = "the cat in the hat";
  $x =~ s/(\w+)/reverse $1/ge;   # $x contains "eht tac ni eht tah"
 
  # convert percentage to decimal
  $x = "A 39% hit rate";
  $x =~ s!(\d+)%!$1/100!e;   # $x contains "A 0.39 hit rate"

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

The split operator

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

 $x = "Calvin and Hobbes";
  @word = split /\s+/, $x;  # $word[0] = 'Calvin'
   # $word[1] = 'and'
   # $word[2] = 'Hobbes'

To extract a comma-delimited list of numbers, use

 $x = "1.618,2.718,   3.142";
  @const = split /,\s*/, $x;  # $const[0] = '1.618'
  # $const[1] = '2.718'
  # $const[2] = '3.142'

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

 $x = "/usr/bin";
  @parts = split m!(/)!, $x;  # $parts[0] = ''
  # $parts[1] = '/'
  # $parts[2] = 'usr'
  # $parts[3] = '/'
  # $parts[4] = 'bin'

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

BUGS

None.

AUTHOR AND COPYRIGHT

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 helpfulcomments.

Page index

NAME
DESCRIPTION
The Guide
BUGS
SEE ALSO
AUTHOR AND COPYRIGHT
- Acknowledgments

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