1PERLREQUICK(1) Perl Programmers Reference Guide PERLREQUICK(1)
2
6 perlrequick - Perl regular expressions quick start
7
9 This page covers the very basics of understanding, creating and using
10 regular expressions ('regexes') in Perl.
11
13 This page assumes you already know things, like what a "pattern" is,
14 and the basic syntax of using them. If you don't, see perlretut.
15 Simple word matching
17 The simplest regex is simply a word, or more generally, a string of
18 characters. A regex consisting of a word matches any string that
19 contains that word:
20 "Hello World" =~ /World/; # matches
22 In this statement, "World" is a regex and the "//" enclosing "/World/"
24 tells Perl to search a string for a match. The operator "=~"
25 associates the string with the regex match and produces a true value if
26 the regex matched, or false if the regex did not match. In our case,
27 "World" matches the second word in "Hello World", so the expression is
28 true. This idea has several variations.
29 Expressions like this are useful in conditionals:
31 print "It matches\n" if "Hello World" =~ /World/;
33 The sense of the match can be reversed by using "!~" operator:
35 print "It doesn't match\n" if "Hello World" !~ /World/;
37 The literal string in the regex can be replaced by a variable:
39 $greeting = "World";
41 print "It matches\n" if "Hello World" =~ /$greeting/;
42 If you're matching against $_, the "$_ =~" part can be omitted:
44 $_ = "Hello World";
46 print "It matches\n" if /World/;
47 Finally, the "//" default delimiters for a match can be changed to
49 arbitrary delimiters by putting an 'm' out front:
50 "Hello World" =~ m!World!; # matches, delimited by '!'
52 "Hello World" =~ m{World}; # matches, note the matching '{}'
53 "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
54 # '/' becomes an ordinary char
55 Regexes must match a part of the string exactly in order for the
57 statement to be true:
58 "Hello World" =~ /world/; # doesn't match, case sensitive
60 "Hello World" =~ /o W/; # matches, ' ' is an ordinary char
61 "Hello World" =~ /World /; # doesn't match, no ' ' at end
62 Perl will always match at the earliest possible point in the string:
64 "Hello World" =~ /o/; # matches 'o' in 'Hello'
66 "That hat is red" =~ /hat/; # matches 'hat' in 'That'
67 Not all characters can be used 'as is' in a match. Some characters,
69 called metacharacters, are reserved for use in regex notation. The
70 metacharacters are
71 {}[]()^$.|*+?\
73 A metacharacter can be matched by putting a backslash before it:
75 "2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
77 "2+2=4" =~ /2\+2/; # matches, \+ is treated like an ordinary +
78 'C:\WIN32' =~ /C:\\WIN/; # matches
79 "/usr/bin/perl" =~ /\/usr\/bin\/perl/; # matches
80 In the last regex, the forward slash '/' is also backslashed, because
82 it is used to delimit the regex.
83 Non-printable ASCII characters are represented by escape sequences.
85 Common examples are "\t" for a tab, "\n" for a newline, and "\r" for a
86 carriage return. Arbitrary bytes are represented by octal escape
87 sequences, e.g., "\033", or hexadecimal escape sequences, e.g., "\x1B":
88 "1000\t2000" =~ m(0\t2) # matches
90 "cat" =~ /\143\x61\x74/ # matches in ASCII, but
91 # a weird way to spell cat
92 Regexes are treated mostly as double-quoted strings, so variable
94 substitution works:
95 $foo = 'house';
97 'cathouse' =~ /cat$foo/; # matches
98 'housecat' =~ /${foo}cat/; # matches
99 With all of the regexes above, if the regex matched anywhere in the
101 string, it was considered a match. To specify where it should match,
102 we would use the anchor metacharacters "^" and "$". The anchor "^"
103 means match at the beginning of the string and the anchor "$" means
104 match at the end of the string, or before a newline at the end of the
105 string. Some examples:
106 "housekeeper" =~ /keeper/; # matches
108 "housekeeper" =~ /^keeper/; # doesn't match
109 "housekeeper" =~ /keeper$/; # matches
110 "housekeeper\n" =~ /keeper$/; # matches
111 "housekeeper" =~ /^housekeeper$/; # matches
112 Using character classes
114 A character class allows a set of possible characters, rather than just
115 a single character, to match at a particular point in a regex.
116 Character classes are denoted by brackets "[...]", with the set of
117 characters to be possibly matched inside. Here are some examples:
118 /cat/; # matches 'cat'
120 /[bcr]at/; # matches 'bat', 'cat', or 'rat'
121 "abc" =~ /[cab]/; # matches 'a'
122 In the last statement, even though 'c' is the first character in the
124 class, the earliest point at which the regex can match is 'a'.
125 /[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
129 The last example shows a match with an 'i' modifier, which makes the
131 match case-insensitive.
132 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 characters
136 for a character class are "-]\^$" and are matched using an escape:
137 /[\]c]def/; # matches ']def' or 'cdef'
139 $x = 'bcr';
140 /[$x]at/; # matches 'bat, 'cat', or 'rat'
141 /[\$x]at/; # matches '$at' or 'xat'
142 /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'
143 The special character '-' acts as a range operator within character
145 classes, so that the unwieldy "[0123456789]" and "[abc...xyz]" become
146 the svelte "[0-9]" and "[a-z]":
147 /item[0-9]/; # matches 'item0' or ... or 'item9'
149 /[0-9a-fA-F]/; # matches a hexadecimal digit
150 If '-' is the first or last character in a character class, it is
152 treated as an ordinary character.
153 The special character "^" in the first position of a character class
155 denotes a negated character class, which matches any character but
156 those in the brackets. Both "[...]" and "[^...]" must match a
157 character, or the match fails. Then
158 /[^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
163 Perl has several abbreviations for common character classes. (These
165 definitions are those that Perl uses in ASCII-safe mode with the "/a"
166 modifier. Otherwise they could match many more non-ASCII Unicode
167 characters as well. See "Backslash sequences" in perlrecharclass for
168 details.)
169 · \d is a digit and represents
171 [0-9]
173 · \s is a whitespace character and represents
175 [\ \t\r\n\f]
177 · \w is a word character (alphanumeric or _) and represents
179 [0-9a-zA-Z_]
181 · \D is a negated \d; it represents any character but a digit
183 [^0-9]
185 · \S is a negated \s; it represents any non-whitespace character
187 [^\s]
189 · \W is a negated \w; it represents any non-word character
191 [^\w]
193 · The period '.' matches any character but "\n"
195 The "\d\s\w\D\S\W" abbreviations can be used both inside and outside of
197 character classes. Here are some in use:
198 /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
200 /[\d\s]/; # matches any digit or whitespace character
201 /\w\W\w/; # matches a word char, followed by a
202 # non-word char, followed by a word char
203 /..rt/; # matches any two chars, followed by 'rt'
204 /end\./; # matches 'end.'
205 /end[.]/; # same thing, matches 'end.'
206 The word anchor "\b" matches a boundary between a word character and a
208 non-word character "\w\W" or "\W\w":
209 $x = "Housecat catenates house and cat";
211 $x =~ /\bcat/; # matches cat in 'catenates'
212 $x =~ /cat\b/; # matches cat in 'housecat'
213 $x =~ /\bcat\b/; # matches 'cat' at end of string
214 In the last example, the end of the string is considered a word
216 boundary.
217 For natural language processing (so that, for example, apostrophes are
219 included in words), use instead "\b{wb}"
220 "don't" =~ / .+? \b{wb} /x; # matches the whole string
222 Matching this or that
224 We can match different character strings with the alternation
225 metacharacter '|'. To match "dog" or "cat", we form the regex
226 "dog|cat". As before, Perl will try to match the regex at the earliest
227 possible point in the string. At each character position, Perl will
228 first try to match the first alternative, "dog". If "dog" doesn't
229 match, Perl will then try the next alternative, "cat". If "cat"
230 doesn't match either, then the match fails and Perl moves to the next
231 position in the string. Some examples:
232 "cats and dogs" =~ /cat|dog|bird/; # matches "cat"
234 "cats and dogs" =~ /dog|cat|bird/; # matches "cat"
235 Even though "dog" is the first alternative in the second regex, "cat"
237 is able to match earlier in the string.
238 "cats" =~ /c|ca|cat|cats/; # matches "c"
240 "cats" =~ /cats|cat|ca|c/; # matches "cats"
241 At a given character position, the first alternative that allows the
243 regex match to succeed will be the one that matches. Here, all the
244 alternatives match at the first string position, so the first matches.
245 Grouping things and hierarchical matching
247 The grouping metacharacters "()" allow a part of a regex to be treated
248 as a single unit. Parts of a regex are grouped by enclosing them in
249 parentheses. The regex "house(cat|keeper)" means match "house"
250 followed by either "cat" or "keeper". Some more examples are
251 /(a|b)b/; # matches 'ab' or 'bb'
253 /(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere
254 /house(cat|)/; # matches either 'housecat' or 'house'
256 /house(cat(s|)|)/; # matches either 'housecats' or 'housecat' or
257 # 'house'. Note groups can be nested.
258 "20" =~ /(19|20|)\d\d/; # matches the null alternative '()\d\d',
260 # because '20\d\d' can't match
261 Extracting matches
263 The grouping metacharacters "()" also allow the extraction of the parts
264 of a string that matched. For each grouping, the part that matched
265 inside goes into the special variables $1, $2, etc. They can be used
266 just as ordinary variables:
267 # extract hours, minutes, seconds
269 $time =~ /(\d\d):(\d\d):(\d\d)/; # match hh:mm:ss format
270 $hours = $1;
271 $minutes = $2;
272 $seconds = $3;
273 In list context, a match "/regex/" with groupings will return the list
275 of matched values "($1,$2,...)". So we could rewrite it as
276 ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);
278 If the groupings in a regex are nested, $1 gets the group with the
280 leftmost opening parenthesis, $2 the next opening parenthesis, etc.
281 For example, here is a complex regex and the matching variables
282 indicated below it:
283 /(ab(cd|ef)((gi)|j))/;
285 1 2 34
286 Associated with the matching variables $1, $2, ... are the
288 backreferences "\g1", "\g2", ... Backreferences are matching variables
289 that can be used inside a regex:
290 /(\w\w\w)\s\g1/; # find sequences like 'the the' in string
292 $1, $2, ... should only be used outside of a regex, and "\g1", "\g2",
294 ... only inside a regex.
295 Matching repetitions
297 The quantifier metacharacters "?", "*", "+", and "{}" allow us to
298 determine the number of repeats of a portion of a regex we consider to
299 be a match. Quantifiers are put immediately after the character,
300 character class, or grouping that we want to specify. They have the
301 following meanings:
302 · "a?" = match 'a' 1 or 0 times
304 · "a*" = match 'a' 0 or more times, i.e., any number of times
306 · "a+" = match 'a' 1 or more times, i.e., at least once
308 · "a{n,m}" = match at least "n" times, but not more than "m" times.
310 · "a{n,}" = match at least "n" or more times
312 · "a{n}" = match exactly "n" times
314 Here are some examples:
316 /[a-z]+\s+\d*/; # match a lowercase word, at least some space, and
318 # any number of digits
319 /(\w+)\s+\g1/; # match doubled words of arbitrary length
320 $year =~ /^\d{2,4}$/; # make sure year is at least 2 but not more
321 # than 4 digits
322 $year =~ /^\d{4}$|^\d{2}$/; # better match; throw out 3 digit dates
323 These quantifiers will try to match as much of the string as possible,
325 while still allowing the regex to match. So we have
326 $x = 'the cat in the hat';
328 $x =~ /^(.*)(at)(.*)$/; # matches,
329 # $1 = 'the cat in the h'
330 # $2 = 'at'
331 # $3 = '' (0 matches)
332 The first quantifier ".*" grabs as much of the string as possible while
334 still having the regex match. The second quantifier ".*" has no string
335 left to it, so it matches 0 times.
336 More matching
338 There are a few more things you might want to know about matching
339 operators. The global modifier "/g" allows the matching operator to
340 match within a string as many times as possible. In scalar context,
341 successive matches against a string will have "/g" jump from match to
342 match, keeping track of position in the string as it goes along. You
343 can get or set the position with the "pos()" function. For example,
344 $x = "cat dog house"; # 3 words
346 while ($x =~ /(\w+)/g) {
347 print "Word is $1, ends at position ", pos $x, "\n";
348 }
349 prints
351 Word is cat, ends at position 3
353 Word is dog, ends at position 7
354 Word is house, ends at position 13
355 A failed match or changing the target string resets the position. If
357 you don't want the position reset after failure to match, add the "/c",
358 as in "/regex/gc".
359 In list context, "/g" returns a list of matched groupings, or if there
361 are no groupings, a list of matches to the whole regex. So
362 @words = ($x =~ /(\w+)/g); # matches,
364 # $word[0] = 'cat'
365 # $word[1] = 'dog'
366 # $word[2] = 'house'
367 Search and replace
369 Search and replace is performed using "s/regex/replacement/modifiers".
370 The "replacement" is a Perl double-quoted string that replaces in the
371 string whatever is matched with the "regex". The operator "=~" is also
372 used here to associate a string with "s///". If matching against $_,
373 the "$_ =~" can be dropped. If there is a match, "s///" returns the
374 number of substitutions made; otherwise it returns false. Here are a
375 few examples:
376 $x = "Time to feed the cat!";
378 $x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!"
379 $y = "'quoted words'";
380 $y =~ s/^'(.*)'$/$1/; # strip single quotes,
381 # $y contains "quoted words"
382 With the "s///" operator, the matched variables $1, $2, etc. are
384 immediately available for use in the replacement expression. With the
385 global modifier, "s///g" will search and replace all occurrences of the
386 regex in the string:
387 $x = "I batted 4 for 4";
389 $x =~ s/4/four/; # $x contains "I batted four for 4"
390 $x = "I batted 4 for 4";
391 $x =~ s/4/four/g; # $x contains "I batted four for four"
392 The non-destructive modifier "s///r" causes the result of the
394 substitution to be returned instead of modifying $_ (or whatever
395 variable the substitute was bound to with "=~"):
396 $x = "I like dogs.";
398 $y = $x =~ s/dogs/cats/r;
399 print "$x $y\n"; # prints "I like dogs. I like cats."
400 $x = "Cats are great.";
402 print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~
403 s/Frogs/Hedgehogs/r, "\n";
404 # prints "Hedgehogs are great."
405 @foo = map { s/[a-z]/X/r } qw(a b c 1 2 3);
407 # @foo is now qw(X X X 1 2 3)
408 The evaluation modifier "s///e" wraps an "eval{...}" around the
410 replacement string and the evaluated result is substituted for the
411 matched substring. Some examples:
412 # reverse all the words in a string
414 $x = "the cat in the hat";
415 $x =~ s/(\w+)/reverse $1/ge; # $x contains "eht tac ni eht tah"
416 # convert percentage to decimal
418 $x = "A 39% hit rate";
419 $x =~ s!(\d+)%!$1/100!e; # $x contains "A 0.39 hit rate"
420 The last example shows that "s///" can use other delimiters, such as
422 "s!!!" and "s{}{}", and even "s{}//". If single quotes are used
423 "s'''", then the regex and replacement are treated as single-quoted
424 strings.
425 The split operator
427 "split /regex/, string" splits "string" into a list of substrings and
428 returns that list. The regex determines the character sequence that
429 "string" is split with respect to. For example, to split a string into
430 words, use
431 $x = "Calvin and Hobbes";
433 @word = split /\s+/, $x; # $word[0] = 'Calvin'
434 # $word[1] = 'and'
435 # $word[2] = 'Hobbes'
436 To extract a comma-delimited list of numbers, use
438 $x = "1.618,2.718, 3.142";
440 @const = split /,\s*/, $x; # $const[0] = '1.618'
441 # $const[1] = '2.718'
442 # $const[2] = '3.142'
443 If the empty regex "//" is used, the string is split into individual
445 characters. If the regex has groupings, then the list produced
446 contains the matched substrings from the groupings as well:
447 $x = "/usr/bin";
449 @parts = split m!(/)!, $x; # $parts[0] = ''
450 # $parts[1] = '/'
451 # $parts[2] = 'usr'
452 # $parts[3] = '/'
453 # $parts[4] = 'bin'
454 Since the first character of $x matched the regex, "split" prepended an
456 empty initial element to the list.
457 "use re 'strict'"
459 New in v5.22, this applies stricter rules than otherwise when compiling
460 regular expression patterns. It can find things that, while legal, may
461 not be what you intended.
462 See 'strict' in re.
464
466 None.
467
469 This is just a quick start guide. For a more in-depth tutorial on
470 regexes, see perlretut and for the reference page, see perlre.
471
473 Copyright (c) 2000 Mark Kvale All rights reserved.
474 This document may be distributed under the same terms as Perl itself.
476 Acknowledgments
478 The author would like to thank Mark-Jason Dominus, Tom Christiansen,
479 Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful
480 comments.
481perl v5.26.3 2018-03-23 PERLREQUICK(1)