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<$_ =~> part can be omitted:
44 print "It matches\n" if /World/;
46 Finally, the C<//> default delimiters for a match can be changed to
47 arbitrary delimiters by putting an C<'m'> out front:
49 "Hello World" =~ m!World!; # matches, delimited by '!'
50 "Hello World" =~ m{World}; # matches, note the matching '{}'
51 "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
52 # '/' becomes an ordinary char
54 Regexes must match a part of the string I<exactly> in order for the
57 "Hello World" =~ /world/; # doesn't match, case sensitive
58 "Hello World" =~ /o W/; # matches, ' ' is an ordinary char
59 "Hello World" =~ /World /; # doesn't match, no ' ' at end
61 Perl will always match at the earliest possible point in the string:
63 "Hello World" =~ /o/; # matches 'o' in 'Hello'
64 "That hat is red" =~ /hat/; # matches 'hat' in 'That'
66 Not all characters can be used 'as is' in a match. Some characters,
67 called B<metacharacters>, are reserved for use in regex notation.
68 The metacharacters are
72 A metacharacter can be matched by putting a backslash before it:
74 "2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
75 "2+2=4" =~ /2\+2/; # matches, \+ is treated like an ordinary +
76 'C:\WIN32' =~ /C:\\WIN/; # matches
77 "/usr/bin/perl" =~ /\/usr\/bin\/perl/; # matches
79 In the last regex, the forward slash C<'/'> is also backslashed,
80 because it is used to delimit the regex.
82 Non-printable ASCII characters are represented by B<escape sequences>.
83 Common examples are C<\t> for a tab, C<\n> for a newline, and C<\r>
84 for a carriage return. Arbitrary bytes are represented by octal
85 escape sequences, e.g., C<\033>, or hexadecimal escape sequences,
88 "1000\t2000" =~ m(0\t2) # matches
89 "cat" =~ /\143\x61\x74/ # matches in ASCII, but a weird way to spell cat
91 Regexes are treated mostly as double-quoted strings, so variable
95 'cathouse' =~ /cat$foo/; # matches
96 'housecat' =~ /${foo}cat/; # matches
98 With all of the regexes above, if the regex matched anywhere in the
99 string, it was considered a match. To specify I<where> it should
100 match, we would use the B<anchor> metacharacters C<^> and C<$>. The
101 anchor C<^> means match at the beginning of the string and the anchor
102 C<$> means match at the end of the string, or before a newline at the
103 end of the string. Some examples:
105 "housekeeper" =~ /keeper/; # matches
106 "housekeeper" =~ /^keeper/; # doesn't match
107 "housekeeper" =~ /keeper$/; # matches
108 "housekeeper\n" =~ /keeper$/; # matches
109 "housekeeper" =~ /^housekeeper$/; # matches
111 =head2 Using character classes
113 A B<character class> allows a set of possible characters, rather than
114 just a single character, to match at a particular point in a regex.
115 Character classes are denoted by brackets C<[...]>, with the set of
116 characters to be possibly matched inside. Here are some examples:
118 /cat/; # matches 'cat'
119 /[bcr]at/; # matches 'bat', 'cat', or 'rat'
120 "abc" =~ /[cab]/; # matches 'a'
122 In the last statement, even though C<'c'> is the first character in
123 the class, the earliest point at which the regex can match is C<'a'>.
125 /[yY][eE][sS]/; # match 'yes' in a case-insensitive way
126 # 'yes', 'Yes', 'YES', etc.
127 /yes/i; # also match 'yes' in a case-insensitive way
129 The last example shows a match with an C<'i'> B<modifier>, which makes
130 the match case-insensitive.
132 Character classes also have ordinary and special characters, but the
133 sets of ordinary and special characters inside a character class are
134 different than those outside a character class. The special
135 characters for a character class are C<-]\^$> and are matched using an
138 /[\]c]def/; # matches ']def' or 'cdef'
140 /[$x]at/; # matches 'bat, 'cat', or 'rat'
141 /[\$x]at/; # matches '$at' or 'xat'
142 /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'
144 The special character C<'-'> acts as a range operator within character
145 classes, so that the unwieldy C<[0123456789]> and C<[abc...xyz]>
146 become the svelte C<[0-9]> and C<[a-z]>:
148 /item[0-9]/; # matches 'item0' or ... or 'item9'
149 /[0-9a-fA-F]/; # matches a hexadecimal digit
151 If C<'-'> is the first or last character in a character class, it is
152 treated as an ordinary character.
154 The special character C<^> in the first position of a character class
155 denotes a B<negated character class>, which matches any character but
156 those in the brackets. Both C<[...]> and C<[^...]> must match a
157 character, or the match fails. Then
159 /[^a]at/; # doesn't match 'aat' or 'at', but matches
160 # all other 'bat', 'cat, '0at', '%at', etc.
161 /[^0-9]/; # matches a non-numeric character
162 /[a^]at/; # matches 'aat' or '^at'; here '^' is ordinary
164 Perl has several abbreviations for common character classes. (These
165 definitions are those that Perl uses in ASCII-safe mode with the C</a> modifier.
166 Otherwise they could match many more non-ASCII Unicode characters as
167 well. See L<perlrecharclass/Backslash sequences> for details.)
173 \d is a digit and represents
179 \s is a whitespace character and represents
185 \w is a word character (alphanumeric or _) and represents
191 \D is a negated \d; it represents any character but a digit
197 \S is a negated \s; it represents any non-whitespace character
203 \W is a negated \w; it represents any non-word character
209 The period '.' matches any character but "\n"
213 The C<\d\s\w\D\S\W> abbreviations can be used both inside and outside
214 of character classes. Here are some in use:
216 /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
217 /[\d\s]/; # matches any digit or whitespace character
218 /\w\W\w/; # matches a word char, followed by a
219 # non-word char, followed by a word char
220 /..rt/; # matches any two chars, followed by 'rt'
221 /end\./; # matches 'end.'
222 /end[.]/; # same thing, matches 'end.'
224 The S<B<word anchor> > C<\b> matches a boundary between a word
225 character and a non-word character C<\w\W> or C<\W\w>:
227 $x = "Housecat catenates house and cat";
228 $x =~ /\bcat/; # matches cat in 'catenates'
229 $x =~ /cat\b/; # matches cat in 'housecat'
230 $x =~ /\bcat\b/; # matches 'cat' at end of string
232 In the last example, the end of the string is considered a word
235 =head2 Matching this or that
237 We can match different character strings with the B<alternation>
238 metacharacter C<'|'>. To match C<dog> or C<cat>, we form the regex
239 C<dog|cat>. As before, Perl will try to match the regex at the
240 earliest possible point in the string. At each character position,
241 Perl will first try to match the first alternative, C<dog>. If
242 C<dog> doesn't match, Perl will then try the next alternative, C<cat>.
243 If C<cat> doesn't match either, then the match fails and Perl moves to
244 the next position in the string. Some examples:
246 "cats and dogs" =~ /cat|dog|bird/; # matches "cat"
247 "cats and dogs" =~ /dog|cat|bird/; # matches "cat"
249 Even though C<dog> is the first alternative in the second regex,
250 C<cat> is able to match earlier in the string.
252 "cats" =~ /c|ca|cat|cats/; # matches "c"
253 "cats" =~ /cats|cat|ca|c/; # matches "cats"
255 At a given character position, the first alternative that allows the
256 regex match to succeed will be the one that matches. Here, all the
257 alternatives match at the first string position, so the first matches.
259 =head2 Grouping things and hierarchical matching
261 The B<grouping> metacharacters C<()> allow a part of a regex to be
262 treated as a single unit. Parts of a regex are grouped by enclosing
263 them in parentheses. The regex C<house(cat|keeper)> means match
264 C<house> followed by either C<cat> or C<keeper>. Some more examples
267 /(a|b)b/; # matches 'ab' or 'bb'
268 /(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere
270 /house(cat|)/; # matches either 'housecat' or 'house'
271 /house(cat(s|)|)/; # matches either 'housecats' or 'housecat' or
272 # 'house'. Note groups can be nested.
274 "20" =~ /(19|20|)\d\d/; # matches the null alternative '()\d\d',
275 # because '20\d\d' can't match
277 =head2 Extracting matches
279 The grouping metacharacters C<()> also allow the extraction of the
280 parts of a string that matched. For each grouping, the part that
281 matched inside goes into the special variables C<$1>, C<$2>, etc.
282 They can be used just as ordinary variables:
284 # extract hours, minutes, seconds
285 $time =~ /(\d\d):(\d\d):(\d\d)/; # match hh:mm:ss format
290 In list context, a match C</regex/> with groupings will return the
291 list of matched values C<($1,$2,...)>. So we could rewrite it as
293 ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);
295 If the groupings in a regex are nested, C<$1> gets the group with the
296 leftmost opening parenthesis, C<$2> the next opening parenthesis,
297 etc. For example, here is a complex regex and the matching variables
300 /(ab(cd|ef)((gi)|j))/;
303 Associated with the matching variables C<$1>, C<$2>, ... are
304 the B<backreferences> C<\g1>, C<\g2>, ... Backreferences are
305 matching variables that can be used I<inside> a regex:
307 /(\w\w\w)\s\g1/; # find sequences like 'the the' in string
309 C<$1>, C<$2>, ... should only be used outside of a regex, and C<\g1>,
310 C<\g2>, ... only inside a regex.
312 =head2 Matching repetitions
314 The B<quantifier> metacharacters C<?>, C<*>, C<+>, and C<{}> allow us
315 to determine the number of repeats of a portion of a regex we
316 consider to be a match. Quantifiers are put immediately after the
317 character, character class, or grouping that we want to specify. They
318 have the following meanings:
324 C<a?> = match 'a' 1 or 0 times
328 C<a*> = match 'a' 0 or more times, i.e., any number of times
332 C<a+> = match 'a' 1 or more times, i.e., at least once
336 C<a{n,m}> = match at least C<n> times, but not more than C<m>
341 C<a{n,}> = match at least C<n> or more times
345 C<a{n}> = match exactly C<n> times
349 Here are some examples:
351 /[a-z]+\s+\d*/; # match a lowercase word, at least some space, and
352 # any number of digits
353 /(\w+)\s+\g1/; # match doubled words of arbitrary length
354 $year =~ /^\d{2,4}$/; # make sure year is at least 2 but not more
356 $year =~ /^\d{4}$|^\d{2}$/; # better match; throw out 3 digit dates
358 These quantifiers will try to match as much of the string as possible,
359 while still allowing the regex to match. So we have
361 $x = 'the cat in the hat';
362 $x =~ /^(.*)(at)(.*)$/; # matches,
363 # $1 = 'the cat in the h'
365 # $3 = '' (0 matches)
367 The first quantifier C<.*> grabs as much of the string as possible
368 while still having the regex match. The second quantifier C<.*> has
369 no string left to it, so it matches 0 times.
373 There are a few more things you might want to know about matching
375 The global modifier C<//g> allows the matching operator to match
376 within a string as many times as possible. In scalar context,
377 successive matches against a string will have C<//g> jump from match
378 to match, keeping track of position in the string as it goes along.
379 You can get or set the position with the C<pos()> function.
382 $x = "cat dog house"; # 3 words
383 while ($x =~ /(\w+)/g) {
384 print "Word is $1, ends at position ", pos $x, "\n";
389 Word is cat, ends at position 3
390 Word is dog, ends at position 7
391 Word is house, ends at position 13
393 A failed match or changing the target string resets the position. If
394 you don't want the position reset after failure to match, add the
395 C<//c>, as in C</regex/gc>.
397 In list context, C<//g> returns a list of matched groupings, or if
398 there are no groupings, a list of matches to the whole regex. So
400 @words = ($x =~ /(\w+)/g); # matches,
405 =head2 Search and replace
407 Search and replace is performed using C<s/regex/replacement/modifiers>.
408 The C<replacement> is a Perl double-quoted string that replaces in the
409 string whatever is matched with the C<regex>. The operator C<=~> is
410 also used here to associate a string with C<s///>. If matching
411 against C<$_>, the S<C<$_ =~>> can be dropped. If there is a match,
412 C<s///> returns the number of substitutions made; otherwise it returns
413 false. Here are a few examples:
415 $x = "Time to feed the cat!";
416 $x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!"
417 $y = "'quoted words'";
418 $y =~ s/^'(.*)'$/$1/; # strip single quotes,
419 # $y contains "quoted words"
421 With the C<s///> operator, the matched variables C<$1>, C<$2>, etc.
422 are immediately available for use in the replacement expression. With
423 the global modifier, C<s///g> will search and replace all occurrences
424 of the regex in the string:
426 $x = "I batted 4 for 4";
427 $x =~ s/4/four/; # $x contains "I batted four for 4"
428 $x = "I batted 4 for 4";
429 $x =~ s/4/four/g; # $x contains "I batted four for four"
431 The non-destructive modifier C<s///r> causes the result of the substitution
432 to be returned instead of modifying C<$_> (or whatever variable the
433 substitute was bound to with C<=~>):
436 $y = $x =~ s/dogs/cats/r;
437 print "$x $y\n"; # prints "I like dogs. I like cats."
439 $x = "Cats are great.";
440 print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~ s/Frogs/Hedgehogs/r, "\n";
441 # prints "Hedgehogs are great."
443 @foo = map { s/[a-z]/X/r } qw(a b c 1 2 3);
444 # @foo is now qw(X X X 1 2 3)
446 The evaluation modifier C<s///e> wraps an C<eval{...}> around the
447 replacement string and the evaluated result is substituted for the
448 matched substring. Some examples:
450 # reverse all the words in a string
451 $x = "the cat in the hat";
452 $x =~ s/(\w+)/reverse $1/ge; # $x contains "eht tac ni eht tah"
454 # convert percentage to decimal
455 $x = "A 39% hit rate";
456 $x =~ s!(\d+)%!$1/100!e; # $x contains "A 0.39 hit rate"
458 The last example shows that C<s///> can use other delimiters, such as
459 C<s!!!> and C<s{}{}>, and even C<s{}//>. If single quotes are used
460 C<s'''>, then the regex and replacement are treated as single-quoted
463 =head2 The split operator
465 C<split /regex/, string> splits C<string> into a list of substrings
466 and returns that list. The regex determines the character sequence
467 that C<string> is split with respect to. For example, to split a
468 string into words, use
470 $x = "Calvin and Hobbes";
471 @word = split /\s+/, $x; # $word[0] = 'Calvin'
473 # $word[2] = 'Hobbes'
475 To extract a comma-delimited list of numbers, use
477 $x = "1.618,2.718, 3.142";
478 @const = split /,\s*/, $x; # $const[0] = '1.618'
479 # $const[1] = '2.718'
480 # $const[2] = '3.142'
482 If the empty regex C<//> is used, the string is split into individual
483 characters. If the regex has groupings, then the list produced contains
484 the matched substrings from the groupings as well:
487 @parts = split m!(/)!, $x; # $parts[0] = ''
493 Since the first character of $x matched the regex, C<split> prepended
494 an empty initial element to the list.
502 This is just a quick start guide. For a more in-depth tutorial on
503 regexes, see L<perlretut> and for the reference page, see L<perlre>.
505 =head1 AUTHOR AND COPYRIGHT
507 Copyright (c) 2000 Mark Kvale
510 This document may be distributed under the same terms as Perl itself.
512 =head2 Acknowledgments
514 The author would like to thank Mark-Jason Dominus, Tom Christiansen,
515 Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful