3 perlrequick - Perl regular expressions quick start
7 This page covers the very basics of understanding, creating and
8 using regular expressions ('regexes') in Perl.
13 =head2 Simple word matching
15 The simplest regex is simply a word, or more generally, a string of
16 characters. A regex consisting of a word matches any string that
19 "Hello World" =~ /World/; # matches
21 In this statement, C<World> is a regex and the C<//> enclosing
22 C</World/> tells Perl to search a string for a match. The operator
23 C<=~> associates the string with the regex match and produces a true
24 value if the regex matched, or false if the regex did not match. In
25 our case, C<World> matches the second word in C<"Hello World">, so the
26 expression is true. This idea has several variations.
28 Expressions like this are useful in conditionals:
30 print "It matches\n" if "Hello World" =~ /World/;
32 The sense of the match can be reversed by using C<!~> operator:
34 print "It doesn't match\n" if "Hello World" !~ /World/;
36 The literal string in the regex can be replaced by a variable:
39 print "It matches\n" if "Hello World" =~ /$greeting/;
41 If you're matching against C<$_>, the C<$_ =~> part can be omitted:
44 print "It matches\n" if /World/;
46 Finally, the C<//> default delimiters for a match can be changed to
47 arbitrary delimiters by putting an C<'m'> out front:
49 "Hello World" =~ m!World!; # matches, delimited by '!'
50 "Hello World" =~ m{World}; # matches, note the matching '{}'
51 "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
52 # '/' becomes an ordinary char
54 Regexes must match a part of the string I<exactly> in order for the
57 "Hello World" =~ /world/; # doesn't match, case sensitive
58 "Hello World" =~ /o W/; # matches, ' ' is an ordinary char
59 "Hello World" =~ /World /; # doesn't match, no ' ' at end
61 Perl will always match at the earliest possible point in the string:
63 "Hello World" =~ /o/; # matches 'o' in 'Hello'
64 "That hat is red" =~ /hat/; # matches 'hat' in 'That'
66 Not all characters can be used 'as is' in a match. Some characters,
67 called B<metacharacters>, are reserved for use in regex notation.
68 The metacharacters are
72 A metacharacter can be matched by putting a backslash before it:
74 "2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
75 "2+2=4" =~ /2\+2/; # matches, \+ is treated like an ordinary +
76 'C:\WIN32' =~ /C:\\WIN/; # matches
77 "/usr/bin/perl" =~ /\/usr\/bin\/perl/; # matches
79 In the last regex, the forward slash C<'/'> is also backslashed,
80 because it is used to delimit the regex.
82 Non-printable ASCII characters are represented by B<escape sequences>.
83 Common examples are C<\t> for a tab, C<\n> for a newline, and C<\r>
84 for a carriage return. Arbitrary bytes are represented by octal
85 escape sequences, e.g., C<\033>, or hexadecimal escape sequences,
88 "1000\t2000" =~ m(0\t2) # matches
89 "cat" =~ /\143\x61\x74/ # matches in ASCII, but
90 # a weird way to spell cat
92 Regexes are treated mostly as double-quoted strings, so variable
96 'cathouse' =~ /cat$foo/; # matches
97 'housecat' =~ /${foo}cat/; # matches
99 With all of the regexes above, if the regex matched anywhere in the
100 string, it was considered a match. To specify I<where> it should
101 match, we would use the B<anchor> metacharacters C<^> and C<$>. The
102 anchor C<^> means match at the beginning of the string and the anchor
103 C<$> means match at the end of the string, or before a newline at the
104 end of the string. Some examples:
106 "housekeeper" =~ /keeper/; # matches
107 "housekeeper" =~ /^keeper/; # doesn't match
108 "housekeeper" =~ /keeper$/; # matches
109 "housekeeper\n" =~ /keeper$/; # matches
110 "housekeeper" =~ /^housekeeper$/; # matches
112 =head2 Using character classes
114 A B<character class> allows a set of possible characters, rather than
115 just a single character, to match at a particular point in a regex.
116 Character classes are denoted by brackets C<[...]>, with the set of
117 characters to be possibly matched inside. Here are some examples:
119 /cat/; # matches 'cat'
120 /[bcr]at/; # matches 'bat', 'cat', or 'rat'
121 "abc" =~ /[cab]/; # matches 'a'
123 In the last statement, even though C<'c'> is the first character in
124 the class, the earliest point at which the regex can match is C<'a'>.
126 /[yY][eE][sS]/; # match 'yes' in a case-insensitive way
127 # 'yes', 'Yes', 'YES', etc.
128 /yes/i; # also match 'yes' in a case-insensitive way
130 The last example shows a match with an C<'i'> B<modifier>, which makes
131 the match case-insensitive.
133 Character classes also have ordinary and special characters, but the
134 sets of ordinary and special characters inside a character class are
135 different than those outside a character class. The special
136 characters for a character class are C<-]\^$> and are matched using an
139 /[\]c]def/; # matches ']def' or 'cdef'
141 /[$x]at/; # matches 'bat, 'cat', or 'rat'
142 /[\$x]at/; # matches '$at' or 'xat'
143 /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'
145 The special character C<'-'> acts as a range operator within character
146 classes, so that the unwieldy C<[0123456789]> and C<[abc...xyz]>
147 become the svelte C<[0-9]> and C<[a-z]>:
149 /item[0-9]/; # matches 'item0' or ... or 'item9'
150 /[0-9a-fA-F]/; # matches a hexadecimal digit
152 If C<'-'> is the first or last character in a character class, it is
153 treated as an ordinary character.
155 The special character C<^> in the first position of a character class
156 denotes a B<negated character class>, which matches any character but
157 those in the brackets. Both C<[...]> and C<[^...]> must match a
158 character, or the match fails. Then
160 /[^a]at/; # doesn't match 'aat' or 'at', but matches
161 # all other 'bat', 'cat, '0at', '%at', etc.
162 /[^0-9]/; # matches a non-numeric character
163 /[a^]at/; # matches 'aat' or '^at'; here '^' is ordinary
165 Perl has several abbreviations for common character classes. (These
166 definitions are those that Perl uses in ASCII-safe mode with the C</a> modifier.
167 Otherwise they could match many more non-ASCII Unicode characters as
168 well. See L<perlrecharclass/Backslash sequences> for details.)
174 \d is a digit and represents
180 \s is a whitespace character and represents
186 \w is a word character (alphanumeric or _) and represents
192 \D is a negated \d; it represents any character but a digit
198 \S is a negated \s; it represents any non-whitespace character
204 \W is a negated \w; it represents any non-word character
210 The period '.' matches any character but "\n"
214 The C<\d\s\w\D\S\W> abbreviations can be used both inside and outside
215 of character classes. Here are some in use:
217 /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
218 /[\d\s]/; # matches any digit or whitespace character
219 /\w\W\w/; # matches a word char, followed by a
220 # non-word char, followed by a word char
221 /..rt/; # matches any two chars, followed by 'rt'
222 /end\./; # matches 'end.'
223 /end[.]/; # same thing, matches 'end.'
225 The S<B<word anchor> > C<\b> matches a boundary between a word
226 character and a non-word character C<\w\W> or C<\W\w>:
228 $x = "Housecat catenates house and cat";
229 $x =~ /\bcat/; # matches cat in 'catenates'
230 $x =~ /cat\b/; # matches cat in 'housecat'
231 $x =~ /\bcat\b/; # matches 'cat' at end of string
233 In the last example, the end of the string is considered a word
236 For natural language processing (so that, for example, apostrophes are
237 included in words), use instead C<\b{wb}>
239 "don't" =~ / .+? \b{wb} /x; # matches the whole string
241 =head2 Matching this or that
243 We can match different character strings with the B<alternation>
244 metacharacter C<'|'>. To match C<dog> or C<cat>, we form the regex
245 C<dog|cat>. As before, Perl will try to match the regex at the
246 earliest possible point in the string. At each character position,
247 Perl will first try to match the first alternative, C<dog>. If
248 C<dog> doesn't match, Perl will then try the next alternative, C<cat>.
249 If C<cat> doesn't match either, then the match fails and Perl moves to
250 the next position in the string. Some examples:
252 "cats and dogs" =~ /cat|dog|bird/; # matches "cat"
253 "cats and dogs" =~ /dog|cat|bird/; # matches "cat"
255 Even though C<dog> is the first alternative in the second regex,
256 C<cat> is able to match earlier in the string.
258 "cats" =~ /c|ca|cat|cats/; # matches "c"
259 "cats" =~ /cats|cat|ca|c/; # matches "cats"
261 At a given character position, the first alternative that allows the
262 regex match to succeed will be the one that matches. Here, all the
263 alternatives match at the first string position, so the first matches.
265 =head2 Grouping things and hierarchical matching
267 The B<grouping> metacharacters C<()> allow a part of a regex to be
268 treated as a single unit. Parts of a regex are grouped by enclosing
269 them in parentheses. The regex C<house(cat|keeper)> means match
270 C<house> followed by either C<cat> or C<keeper>. Some more examples
273 /(a|b)b/; # matches 'ab' or 'bb'
274 /(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere
276 /house(cat|)/; # matches either 'housecat' or 'house'
277 /house(cat(s|)|)/; # matches either 'housecats' or 'housecat' or
278 # 'house'. Note groups can be nested.
280 "20" =~ /(19|20|)\d\d/; # matches the null alternative '()\d\d',
281 # because '20\d\d' can't match
283 =head2 Extracting matches
285 The grouping metacharacters C<()> also allow the extraction of the
286 parts of a string that matched. For each grouping, the part that
287 matched inside goes into the special variables C<$1>, C<$2>, etc.
288 They can be used just as ordinary variables:
290 # extract hours, minutes, seconds
291 $time =~ /(\d\d):(\d\d):(\d\d)/; # match hh:mm:ss format
296 In list context, a match C</regex/> with groupings will return the
297 list of matched values C<($1,$2,...)>. So we could rewrite it as
299 ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);
301 If the groupings in a regex are nested, C<$1> gets the group with the
302 leftmost opening parenthesis, C<$2> the next opening parenthesis,
303 etc. For example, here is a complex regex and the matching variables
306 /(ab(cd|ef)((gi)|j))/;
309 Associated with the matching variables C<$1>, C<$2>, ... are
310 the B<backreferences> C<\g1>, C<\g2>, ... Backreferences are
311 matching variables that can be used I<inside> a regex:
313 /(\w\w\w)\s\g1/; # find sequences like 'the the' in string
315 C<$1>, C<$2>, ... should only be used outside of a regex, and C<\g1>,
316 C<\g2>, ... only inside a regex.
318 =head2 Matching repetitions
320 The B<quantifier> metacharacters C<?>, C<*>, C<+>, and C<{}> allow us
321 to determine the number of repeats of a portion of a regex we
322 consider to be a match. Quantifiers are put immediately after the
323 character, character class, or grouping that we want to specify. They
324 have the following meanings:
330 C<a?> = match 'a' 1 or 0 times
334 C<a*> = match 'a' 0 or more times, i.e., any number of times
338 C<a+> = match 'a' 1 or more times, i.e., at least once
342 C<a{n,m}> = match at least C<n> times, but not more than C<m>
347 C<a{n,}> = match at least C<n> or more times
351 C<a{n}> = match exactly C<n> times
355 Here are some examples:
357 /[a-z]+\s+\d*/; # match a lowercase word, at least some space, and
358 # any number of digits
359 /(\w+)\s+\g1/; # match doubled words of arbitrary length
360 $year =~ /^\d{2,4}$/; # make sure year is at least 2 but not more
362 $year =~ /^\d{4}$|^\d{2}$/; # better match; throw out 3 digit dates
364 These quantifiers will try to match as much of the string as possible,
365 while still allowing the regex to match. So we have
367 $x = 'the cat in the hat';
368 $x =~ /^(.*)(at)(.*)$/; # matches,
369 # $1 = 'the cat in the h'
371 # $3 = '' (0 matches)
373 The first quantifier C<.*> grabs as much of the string as possible
374 while still having the regex match. The second quantifier C<.*> has
375 no string left to it, so it matches 0 times.
379 There are a few more things you might want to know about matching
381 The global modifier C<//g> allows the matching operator to match
382 within a string as many times as possible. In scalar context,
383 successive matches against a string will have C<//g> jump from match
384 to match, keeping track of position in the string as it goes along.
385 You can get or set the position with the C<pos()> function.
388 $x = "cat dog house"; # 3 words
389 while ($x =~ /(\w+)/g) {
390 print "Word is $1, ends at position ", pos $x, "\n";
395 Word is cat, ends at position 3
396 Word is dog, ends at position 7
397 Word is house, ends at position 13
399 A failed match or changing the target string resets the position. If
400 you don't want the position reset after failure to match, add the
401 C<//c>, as in C</regex/gc>.
403 In list context, C<//g> returns a list of matched groupings, or if
404 there are no groupings, a list of matches to the whole regex. So
406 @words = ($x =~ /(\w+)/g); # matches,
411 =head2 Search and replace
413 Search and replace is performed using C<s/regex/replacement/modifiers>.
414 The C<replacement> is a Perl double-quoted string that replaces in the
415 string whatever is matched with the C<regex>. The operator C<=~> is
416 also used here to associate a string with C<s///>. If matching
417 against C<$_>, the S<C<$_ =~>> can be dropped. If there is a match,
418 C<s///> returns the number of substitutions made; otherwise it returns
419 false. Here are a few examples:
421 $x = "Time to feed the cat!";
422 $x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!"
423 $y = "'quoted words'";
424 $y =~ s/^'(.*)'$/$1/; # strip single quotes,
425 # $y contains "quoted words"
427 With the C<s///> operator, the matched variables C<$1>, C<$2>, etc.
428 are immediately available for use in the replacement expression. With
429 the global modifier, C<s///g> will search and replace all occurrences
430 of the regex in the string:
432 $x = "I batted 4 for 4";
433 $x =~ s/4/four/; # $x contains "I batted four for 4"
434 $x = "I batted 4 for 4";
435 $x =~ s/4/four/g; # $x contains "I batted four for four"
437 The non-destructive modifier C<s///r> causes the result of the substitution
438 to be returned instead of modifying C<$_> (or whatever variable the
439 substitute was bound to with C<=~>):
442 $y = $x =~ s/dogs/cats/r;
443 print "$x $y\n"; # prints "I like dogs. I like cats."
445 $x = "Cats are great.";
446 print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~
447 s/Frogs/Hedgehogs/r, "\n";
448 # prints "Hedgehogs are great."
450 @foo = map { s/[a-z]/X/r } qw(a b c 1 2 3);
451 # @foo is now qw(X X X 1 2 3)
453 The evaluation modifier C<s///e> wraps an C<eval{...}> around the
454 replacement string and the evaluated result is substituted for the
455 matched substring. Some examples:
457 # reverse all the words in a string
458 $x = "the cat in the hat";
459 $x =~ s/(\w+)/reverse $1/ge; # $x contains "eht tac ni eht tah"
461 # convert percentage to decimal
462 $x = "A 39% hit rate";
463 $x =~ s!(\d+)%!$1/100!e; # $x contains "A 0.39 hit rate"
465 The last example shows that C<s///> can use other delimiters, such as
466 C<s!!!> and C<s{}{}>, and even C<s{}//>. If single quotes are used
467 C<s'''>, then the regex and replacement are treated as single-quoted
470 =head2 The split operator
472 C<split /regex/, string> splits C<string> into a list of substrings
473 and returns that list. The regex determines the character sequence
474 that C<string> is split with respect to. For example, to split a
475 string into words, use
477 $x = "Calvin and Hobbes";
478 @word = split /\s+/, $x; # $word[0] = 'Calvin'
480 # $word[2] = 'Hobbes'
482 To extract a comma-delimited list of numbers, use
484 $x = "1.618,2.718, 3.142";
485 @const = split /,\s*/, $x; # $const[0] = '1.618'
486 # $const[1] = '2.718'
487 # $const[2] = '3.142'
489 If the empty regex C<//> is used, the string is split into individual
490 characters. If the regex has groupings, then the list produced contains
491 the matched substrings from the groupings as well:
494 @parts = split m!(/)!, $x; # $parts[0] = ''
500 Since the first character of $x matched the regex, C<split> prepended
501 an empty initial element to the list.
503 =head2 C<use re 'strict'>
505 New in v5.22, this applies stricter rules than otherwise when compiling
506 regular expression patterns. It can find things that, while legal, may
507 not be what you intended.
509 See L<'strict' in re|re/'strict' mode>.
517 This is just a quick start guide. For a more in-depth tutorial on
518 regexes, see L<perlretut> and for the reference page, see L<perlre>.
520 =head1 AUTHOR AND COPYRIGHT
522 Copyright (c) 2000 Mark Kvale
525 This document may be distributed under the same terms as Perl itself.
527 =head2 Acknowledgments
529 The author would like to thank Mark-Jason Dominus, Tom Christiansen,
530 Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful