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<$_ =~> part can be omitted:
44 print "It matches\n" if /World/;
46 Finally, the C<//> default delimiters for a match can be changed to
47 arbitrary delimiters by putting an C<'m'> out front:
49 "Hello World" =~ m!World!; # matches, delimited by '!'
50 "Hello World" =~ m{World}; # matches, note the matching '{}'
51 "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
52 # '/' becomes an ordinary char
54 Regexes must match a part of the string I<exactly> in order for the
57 "Hello World" =~ /world/; # doesn't match, case sensitive
58 "Hello World" =~ /o W/; # matches, ' ' is an ordinary char
59 "Hello World" =~ /World /; # doesn't match, no ' ' at end
61 perl will always match at the earliest possible point in the string:
63 "Hello World" =~ /o/; # matches 'o' in 'Hello'
64 "That hat is red" =~ /hat/; # matches 'hat' in 'That'
66 Not all characters can be used 'as is' in a match. Some characters,
67 called B<metacharacters>, are reserved for use in regex notation.
68 The metacharacters are
72 A metacharacter can be matched by putting a backslash before it:
74 "2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
75 "2+2=4" =~ /2\+2/; # matches, \+ is treated like an ordinary +
76 'C:\WIN32' =~ /C:\\WIN/; # matches
77 "/usr/bin/perl" =~ /\/usr\/local\/bin\/perl/; # matches
79 In the last regex, the forward slash C<'/'> is also backslashed,
80 because it is used to delimit the regex.
82 Non-printable ASCII characters are represented by B<escape sequences>.
83 Common examples are C<\t> for a tab, C<\n> for a newline, and C<\r>
84 for a carriage return. Arbitrary bytes are represented by octal
85 escape sequences, e.g., C<\033>, or hexadecimal escape sequences,
88 "1000\t2000" =~ m(0\t2) # matches
89 "cat" =~ /\143\x61\x74/ # matches, but a weird way to spell cat
91 Regexes are treated mostly as double quoted strings, so variable
95 'cathouse' =~ /cat$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:
169 \d is a digit and represents [0-9]
172 \s is a whitespace character and represents [\ \t\r\n\f]
175 \w is a word character (alphanumeric or _) and represents [0-9a-zA-Z_]
178 \D is a negated \d; it represents any character but a digit [^0-9]
181 \S is a negated \s; it represents any non-whitespace character [^\s]
184 \W is a negated \w; it represents any non-word character [^\w]
187 The period '.' matches any character but "\n"
191 The C<\d\s\w\D\S\W> abbreviations can be used both inside and outside
192 of character classes. Here are some in use:
194 /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
195 /[\d\s]/; # matches any digit or whitespace character
196 /\w\W\w/; # matches a word char, followed by a
197 # non-word char, followed by a word char
198 /..rt/; # matches any two chars, followed by 'rt'
199 /end\./; # matches 'end.'
200 /end[.]/; # same thing, matches 'end.'
202 The S<B<word anchor> > C<\b> matches a boundary between a word
203 character and a non-word character C<\w\W> or C<\W\w>:
205 $x = "Housecat catenates house and cat";
206 $x =~ /\bcat/; # matches cat in 'catenates'
207 $x =~ /cat\b/; # matches cat in 'housecat'
208 $x =~ /\bcat\b/; # matches 'cat' at end of string
210 In the last example, the end of the string is considered a word
213 =head2 Matching this or that
215 We can match match different character strings with the B<alternation>
216 metacharacter C<'|'>. To match C<dog> or C<cat>, we form the regex
217 C<dog|cat>. As before, perl will try to match the regex at the
218 earliest possible point in the string. At each character position,
219 perl will first try to match the the first alternative, C<dog>. If
220 C<dog> doesn't match, perl will then try the next alternative, C<cat>.
221 If C<cat> doesn't match either, then the match fails and perl moves to
222 the next position in the string. Some examples:
224 "cats and dogs" =~ /cat|dog|bird/; # matches "cat"
225 "cats and dogs" =~ /dog|cat|bird/; # matches "cat"
227 Even though C<dog> is the first alternative in the second regex,
228 C<cat> is able to match earlier in the string.
230 "cats" =~ /c|ca|cat|cats/; # matches "c"
231 "cats" =~ /cats|cat|ca|c/; # matches "cats"
233 At a given character position, the first alternative that allows the
234 regex match to succeed wil be the one that matches. Here, all the
235 alternatives match at the first string position, so th first matches.
237 =head2 Grouping things and hierarchical matching
239 The B<grouping> metacharacters C<()> allow a part of a regex to be
240 treated as a single unit. Parts of a regex are grouped by enclosing
241 them in parentheses. The regex C<house(cat|keeper)> means match
242 C<house> followed by either C<cat> or C<keeper>. Some more examples
245 /(a|b)b/; # matches 'ab' or 'bb'
246 /(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere
248 /house(cat|)/; # matches either 'housecat' or 'house'
249 /house(cat(s|)|)/; # matches either 'housecats' or 'housecat' or
250 # 'house'. Note groups can be nested.
252 "20" =~ /(19|20|)\d\d/; # matches the null alternative '()\d\d',
253 # because '20\d\d' can't match
255 =head2 Extracting matches
257 The grouping metacharacters C<()> also allow the extraction of the
258 parts of a string that matched. For each grouping, the part that
259 matched inside goes into the special variables C<$1>, C<$2>, etc.
260 They can be used just as ordinary variables:
262 # extract hours, minutes, seconds
263 $time =~ /(\d\d):(\d\d):(\d\d)/; # match hh:mm:ss format
268 In list context, a match C</regex/> with groupings will return the
269 list of matched values C<($1,$2,...)>. So we could rewrite it as
271 ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);
273 If the groupings in a regex are nested, C<$1> gets the group with the
274 leftmost opening parenthesis, C<$2> the next opening parenthesis,
275 etc. For example, here is a complex regex and the matching variables
278 /(ab(cd|ef)((gi)|j))/;
281 Associated with the matching variables C<$1>, C<$2>, ... are
282 the B<backreferences> C<\1>, C<\2>, ... Backreferences are
283 matching variables that can be used I<inside> a regex:
285 /(\w\w\w)\s\1/; # find sequences like 'the the' in string
287 C<$1>, C<$2>, ... should only be used outside of a regex, and C<\1>,
288 C<\2>, ... only inside a regex.
290 =head2 Matching repetitions
292 The B<quantifier> metacharacters C<?>, C<*>, C<+>, and C<{}> allow us
293 to determine the number of repeats of a portion of a regex we
294 consider to be a match. Quantifiers are put immediately after the
295 character, character class, or grouping that we want to specify. They
296 have the following meanings:
300 =item * C<a?> = match 'a' 1 or 0 times
302 =item * C<a*> = match 'a' 0 or more times, i.e., any number of times
304 =item * C<a+> = match 'a' 1 or more times, i.e., at least once
306 =item * C<a{n,m}> = match at least C<n> times, but not more than C<m>
309 =item * C<a{n,}> = match at least C<n> or more times
311 =item * C<a{n}> = match exactly C<n> times
315 Here are some examples:
317 /[a-z]+\s+\d*/; # match a lowercase word, at least some space, and
318 # any number of digits
319 /(\w+)\s+\1/; # match doubled words of arbitrary length
320 $year =~ /\d{2,4}/; # make sure year is at least 2 but not more
322 $year =~ /\d{4}|\d{2}/; # better match; throw out 3 digit dates
324 These quantifiers will try to match as much of the string as possible,
325 while still allowing the regex to match. So we have
327 $x = 'the cat in the hat';
328 $x =~ /^(.*)(at)(.*)$/; # matches,
329 # $1 = 'the cat in the h'
331 # $3 = '' (0 matches)
333 The first quantifier C<.*> grabs as much of the string as possible
334 while still having the regex match. The second quantifier C<.*> has
335 no string left to it, so it matches 0 times.
339 There are a few more things you might want to know about matching
340 operators. In the code
347 perl has to re-evaluate C<$pattern> each time through the loop. If
348 C<$pattern> won't be changing, use the C<//o> modifier, to only
349 perform variable substitutions once. If you don't want any
350 substitutions at all, use the special delimiter C<m''>:
353 m'$pattern'; # matches '$pattern', not 'Seuss'
355 The global modifier C<//g> allows the matching operator to match
356 within a string as many times as possible. In scalar context,
357 successive matches against a string will have C<//g> jump from match
358 to match, keeping track of position in the string as it goes along.
359 You can get or set the position with the C<pos()> function.
362 $x = "cat dog house"; # 3 words
363 while ($x =~ /(\w+)/g) {
364 print "Word is $1, ends at position ", pos $x, "\n";
369 Word is cat, ends at position 3
370 Word is dog, ends at position 7
371 Word is house, ends at position 13
373 A failed match or changing the target string resets the position. If
374 you don't want the position reset after failure to match, add the
375 C<//c>, as in C</regex/gc>.
377 In list context, C<//g> returns a list of matched groupings, or if
378 there are no groupings, a list of matches to the whole regex. So
380 @words = ($x =~ /(\w+)/g); # matches,
385 =head2 Search and replace
387 Search and replace is performed using C<s/regex/replacement/modifiers>.
388 The C<replacement> is a Perl double quoted string that replaces in the
389 string whatever is matched with the C<regex>. The operator C<=~> is
390 also used here to associate a string with C<s///>. If matching
391 against C<$_>, the S<C<$_ =~> > can be dropped. If there is a match,
392 C<s///> returns the number of substitutions made, otherwise it returns
393 false. Here are a few examples:
395 $x = "Time to feed the cat!";
396 $x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!"
397 $y = "'quoted words'";
398 $y =~ s/^'(.*)'$/$1/; # strip single quotes,
399 # $y contains "quoted words"
401 With the C<s///> operator, the matched variables C<$1>, C<$2>, etc.
402 are immediately available for use in the replacement expression. With
403 the global modifier, C<s///g> will search and replace all occurrences
404 of the regex in the string:
406 $x = "I batted 4 for 4";
407 $x =~ s/4/four/; # $x contains "I batted four for 4"
408 $x = "I batted 4 for 4";
409 $x =~ s/4/four/g; # $x contains "I batted four for four"
411 The evaluation modifier C<s///e> wraps an C<eval{...}> around the
412 replacement string and the evaluated result is substituted for the
413 matched substring. Some examples:
415 # reverse all the words in a string
416 $x = "the cat in the hat";
417 $x =~ s/(\w+)/reverse $1/ge; # $x contains "eht tac ni eht tah"
419 # convert percentage to decimal
420 $x = "A 39% hit rate";
421 $x =~ s!(\d+)%!$1/100!e; # $x contains "A 0.39 hit rate"
423 The last example shows that C<s///> can use other delimiters, such as
424 C<s!!!> and C<s{}{}>, and even C<s{}//>. If single quotes are used
425 C<s'''>, then the regex and replacement are treated as single quoted
428 =head2 The split operator
430 C<split /regex/, string> splits C<string> into a list of substrings
431 and returns that list. The regex determines the character sequence
432 that C<string> is split with respect to. For example, to split a
433 string into words, use
435 $x = "Calvin and Hobbes";
436 @word = split /\s+/, $x; # $word[0] = 'Calvin'
438 # $word[2] = 'Hobbes'
440 To extract a comma-delimited list of numbers, use
442 $x = "1.618,2.718, 3.142";
443 @const = split /,\s*/, $x; # $const[0] = '1.618'
444 # $const[1] = '2.718'
445 # $const[2] = '3.142'
447 If the empty regex C<//> is used, the string is split into individual
448 characters. If the regex has groupings, then list produced contains
449 the matched substrings from the groupings as well:
452 @parts = split m!(/)!, $x; # $parts[0] = ''
458 Since the first character of $x matched the regex, C<split> prepended
459 an empty initial element to the list.
467 This is just a quick start guide. For a more in-depth tutorial on
468 regexes, see L<perlretut> and for the reference page, see L<perlre>.
470 =head1 AUTHOR AND COPYRIGHT
472 Copyright (c) 2000 Mark Kvale
475 This document may be distributed under the same terms as Perl itself.
477 =head2 Acknowledgments
479 The author would like to thank Mark-Jason Dominus, Tom Christiansen,
480 Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful