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1 | =head1 NAME |
2 | ||
3 | perlrequick - Perl regular expressions quick start | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | This page covers the very basics of understanding, creating and | |
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8 | using regular expressions ('regexes') in Perl. |
9 | ||
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10 | |
11 | =head1 The Guide | |
12 | ||
13 | =head2 Simple word matching | |
14 | ||
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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 | |
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17 | contains that word: |
18 | ||
19 | "Hello World" =~ /World/; # matches | |
20 | ||
6425a278 | 21 | In this statement, C<World> is a regex and the C<//> enclosing |
47f9c88b | 22 | C</World/> tells perl to search a string for a match. The operator |
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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 | |
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25 | our case, C<World> matches the second word in C<"Hello World">, so the |
26 | expression is true. This idea has several variations. | |
27 | ||
28 | Expressions like this are useful in conditionals: | |
29 | ||
30 | print "It matches\n" if "Hello World" =~ /World/; | |
31 | ||
32 | The sense of the match can be reversed by using C<!~> operator: | |
33 | ||
34 | print "It doesn't match\n" if "Hello World" !~ /World/; | |
35 | ||
6425a278 | 36 | The literal string in the regex can be replaced by a variable: |
47f9c88b GS |
37 | |
38 | $greeting = "World"; | |
39 | print "It matches\n" if "Hello World" =~ /$greeting/; | |
40 | ||
41 | If you're matching against C<$_>, the C<$_ =~> part can be omitted: | |
42 | ||
43 | $_ = "Hello World"; | |
44 | print "It matches\n" if /World/; | |
45 | ||
46 | Finally, the C<//> default delimiters for a match can be changed to | |
47 | arbitrary delimiters by putting an C<'m'> out front: | |
48 | ||
49 | "Hello World" =~ m!World!; # matches, delimited by '!' | |
50 | "Hello World" =~ m{World}; # matches, note the matching '{}' | |
51 | "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin', | |
52 | # '/' becomes an ordinary char | |
53 | ||
6425a278 | 54 | Regexes must match a part of the string I<exactly> in order for the |
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55 | statement to be true: |
56 | ||
57 | "Hello World" =~ /world/; # doesn't match, case sensitive | |
58 | "Hello World" =~ /o W/; # matches, ' ' is an ordinary char | |
59 | "Hello World" =~ /World /; # doesn't match, no ' ' at end | |
60 | ||
61 | perl will always match at the earliest possible point in the string: | |
62 | ||
63 | "Hello World" =~ /o/; # matches 'o' in 'Hello' | |
64 | "That hat is red" =~ /hat/; # matches 'hat' in 'That' | |
65 | ||
66 | Not all characters can be used 'as is' in a match. Some characters, | |
6425a278 | 67 | called B<metacharacters>, are reserved for use in regex notation. |
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68 | The metacharacters are |
69 | ||
70 | {}[]()^$.|*+?\ | |
71 | ||
72 | A metacharacter can be matched by putting a backslash before it: | |
73 | ||
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 | |
78 | ||
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79 | In the last regex, the forward slash C<'/'> is also backslashed, |
80 | because it is used to delimit the regex. | |
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81 | |
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, | |
86 | e.g., C<\x1B>: | |
87 | ||
88 | "1000\t2000" =~ m(0\t2) # matches | |
89 | "cat" =~ /\143\x61\x74/ # matches, but a weird way to spell cat | |
90 | ||
6425a278 | 91 | Regexes are treated mostly as double quoted strings, so variable |
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92 | substitution works: |
93 | ||
94 | $foo = 'house'; | |
95 | 'cathouse' =~ /cat$foo/; # matches | |
96 | 'housecat' =~ /${foo}cat/; # matches | |
97 | ||
6425a278 | 98 | With all of the regexes above, if the regex matched anywhere in the |
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99 | string, it was considered a match. To specify I<where> it should |
100 | match, we would use the B<anchor> metacharacters C<^> and C<$>. The | |
101 | anchor C<^> means match at the beginning of the string and the anchor | |
102 | C<$> means match at the end of the string, or before a newline at the | |
103 | end of the string. Some examples: | |
104 | ||
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105 | "housekeeper" =~ /keeper/; # matches |
106 | "housekeeper" =~ /^keeper/; # doesn't match | |
107 | "housekeeper" =~ /keeper$/; # matches | |
108 | "housekeeper\n" =~ /keeper$/; # matches | |
109 | "housekeeper" =~ /^housekeeper$/; # matches | |
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110 | |
111 | =head2 Using character classes | |
112 | ||
113 | A B<character class> allows a set of possible characters, rather than | |
6425a278 | 114 | just a single character, to match at a particular point in a regex. |
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115 | Character classes are denoted by brackets C<[...]>, with the set of |
116 | characters to be possibly matched inside. Here are some examples: | |
117 | ||
118 | /cat/; # matches 'cat' | |
6425a278 | 119 | /[bcr]at/; # matches 'bat', 'cat', or 'rat' |
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120 | "abc" =~ /[cab]/; # matches 'a' |
121 | ||
122 | In the last statement, even though C<'c'> is the first character in | |
6425a278 | 123 | the class, the earliest point at which the regex can match is C<'a'>. |
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124 | |
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 | |
128 | ||
129 | The last example shows a match with an C<'i'> B<modifier>, which makes | |
130 | the match case-insensitive. | |
131 | ||
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 | |
136 | escape: | |
137 | ||
138 | /[\]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 | ||
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]>: | |
147 | ||
148 | /item[0-9]/; # matches 'item0' or ... or 'item9' | |
149 | /[0-9a-fA-F]/; # matches a hexadecimal digit | |
150 | ||
151 | If C<'-'> is the first or last character in a character class, it is | |
152 | treated as an ordinary character. | |
153 | ||
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 | |
6425a278 | 156 | those in the brackets. Both C<[...]> and C<[^...]> must match a |
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157 | character, or the match fails. Then |
158 | ||
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 | |
163 | ||
164 | Perl has several abbreviations for common character classes: | |
165 | ||
166 | =over 4 | |
167 | ||
168 | =item * | |
551e1d92 | 169 | |
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170 | \d is a digit and represents [0-9] |
171 | ||
172 | =item * | |
551e1d92 | 173 | |
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174 | \s is a whitespace character and represents [\ \t\r\n\f] |
175 | ||
176 | =item * | |
551e1d92 | 177 | |
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178 | \w is a word character (alphanumeric or _) and represents [0-9a-zA-Z_] |
179 | ||
180 | =item * | |
551e1d92 | 181 | |
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182 | \D is a negated \d; it represents any character but a digit [^0-9] |
183 | ||
184 | =item * | |
551e1d92 | 185 | |
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186 | \S is a negated \s; it represents any non-whitespace character [^\s] |
187 | ||
188 | =item * | |
551e1d92 | 189 | |
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190 | \W is a negated \w; it represents any non-word character [^\w] |
191 | ||
192 | =item * | |
551e1d92 | 193 | |
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194 | The period '.' matches any character but "\n" |
195 | ||
196 | =back | |
197 | ||
198 | The C<\d\s\w\D\S\W> abbreviations can be used both inside and outside | |
199 | of character classes. Here are some in use: | |
200 | ||
201 | /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format | |
202 | /[\d\s]/; # matches any digit or whitespace character | |
203 | /\w\W\w/; # matches a word char, followed by a | |
204 | # non-word char, followed by a word char | |
205 | /..rt/; # matches any two chars, followed by 'rt' | |
206 | /end\./; # matches 'end.' | |
207 | /end[.]/; # same thing, matches 'end.' | |
208 | ||
209 | The S<B<word anchor> > C<\b> matches a boundary between a word | |
210 | character and a non-word character C<\w\W> or C<\W\w>: | |
211 | ||
212 | $x = "Housecat catenates house and cat"; | |
213 | $x =~ /\bcat/; # matches cat in 'catenates' | |
214 | $x =~ /cat\b/; # matches cat in 'housecat' | |
215 | $x =~ /\bcat\b/; # matches 'cat' at end of string | |
216 | ||
217 | In the last example, the end of the string is considered a word | |
218 | boundary. | |
219 | ||
220 | =head2 Matching this or that | |
221 | ||
da75cd15 | 222 | We can match different character strings with the B<alternation> |
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223 | metacharacter C<'|'>. To match C<dog> or C<cat>, we form the regex |
224 | C<dog|cat>. As before, perl will try to match the regex at the | |
47f9c88b | 225 | earliest possible point in the string. At each character position, |
da75cd15 | 226 | perl will first try to match the first alternative, C<dog>. If |
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227 | C<dog> doesn't match, perl will then try the next alternative, C<cat>. |
228 | If C<cat> doesn't match either, then the match fails and perl moves to | |
229 | the next position in the string. Some examples: | |
230 | ||
231 | "cats and dogs" =~ /cat|dog|bird/; # matches "cat" | |
232 | "cats and dogs" =~ /dog|cat|bird/; # matches "cat" | |
233 | ||
6425a278 | 234 | Even though C<dog> is the first alternative in the second regex, |
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235 | C<cat> is able to match earlier in the string. |
236 | ||
237 | "cats" =~ /c|ca|cat|cats/; # matches "c" | |
238 | "cats" =~ /cats|cat|ca|c/; # matches "cats" | |
239 | ||
240 | At a given character position, the first alternative that allows the | |
210b36aa | 241 | regex match to succeed will be the one that matches. Here, all the |
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242 | alternatives match at the first string position, so th first matches. |
243 | ||
244 | =head2 Grouping things and hierarchical matching | |
245 | ||
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246 | The B<grouping> metacharacters C<()> allow a part of a regex to be |
247 | treated as a single unit. Parts of a regex are grouped by enclosing | |
248 | them in parentheses. The regex C<house(cat|keeper)> means match | |
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249 | C<house> followed by either C<cat> or C<keeper>. Some more examples |
250 | are | |
251 | ||
252 | /(a|b)b/; # matches 'ab' or 'bb' | |
253 | /(^a|b)c/; # matches 'ac' at start of string or 'bc' anywhere | |
254 | ||
255 | /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 | ||
259 | "20" =~ /(19|20|)\d\d/; # matches the null alternative '()\d\d', | |
260 | # because '20\d\d' can't match | |
261 | ||
262 | =head2 Extracting matches | |
263 | ||
264 | The grouping metacharacters C<()> also allow the extraction of the | |
265 | parts of a string that matched. For each grouping, the part that | |
266 | matched inside goes into the special variables C<$1>, C<$2>, etc. | |
267 | They can be used just as ordinary variables: | |
268 | ||
269 | # extract hours, minutes, seconds | |
270 | $time =~ /(\d\d):(\d\d):(\d\d)/; # match hh:mm:ss format | |
271 | $hours = $1; | |
272 | $minutes = $2; | |
273 | $seconds = $3; | |
274 | ||
6425a278 | 275 | In list context, a match C</regex/> with groupings will return the |
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276 | list of matched values C<($1,$2,...)>. So we could rewrite it as |
277 | ||
278 | ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/); | |
279 | ||
6425a278 | 280 | If the groupings in a regex are nested, C<$1> gets the group with the |
47f9c88b | 281 | leftmost opening parenthesis, C<$2> the next opening parenthesis, |
6425a278 | 282 | etc. For example, here is a complex regex and the matching variables |
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283 | indicated below it: |
284 | ||
285 | /(ab(cd|ef)((gi)|j))/; | |
286 | 1 2 34 | |
287 | ||
288 | Associated with the matching variables C<$1>, C<$2>, ... are | |
289 | the B<backreferences> C<\1>, C<\2>, ... Backreferences are | |
6425a278 | 290 | matching variables that can be used I<inside> a regex: |
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291 | |
292 | /(\w\w\w)\s\1/; # find sequences like 'the the' in string | |
293 | ||
6425a278 GS |
294 | C<$1>, C<$2>, ... should only be used outside of a regex, and C<\1>, |
295 | C<\2>, ... only inside a regex. | |
47f9c88b GS |
296 | |
297 | =head2 Matching repetitions | |
298 | ||
299 | The B<quantifier> metacharacters C<?>, C<*>, C<+>, and C<{}> allow us | |
6425a278 | 300 | to determine the number of repeats of a portion of a regex we |
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301 | consider to be a match. Quantifiers are put immediately after the |
302 | character, character class, or grouping that we want to specify. They | |
303 | have the following meanings: | |
304 | ||
305 | =over 4 | |
306 | ||
cb49b31f RB |
307 | =item * |
308 | ||
309 | C<a?> = match 'a' 1 or 0 times | |
310 | ||
311 | =item * | |
312 | ||
313 | C<a*> = match 'a' 0 or more times, i.e., any number of times | |
314 | ||
315 | =item * | |
47f9c88b | 316 | |
cb49b31f | 317 | C<a+> = match 'a' 1 or more times, i.e., at least once |
47f9c88b | 318 | |
cb49b31f | 319 | =item * |
47f9c88b | 320 | |
cb49b31f | 321 | C<a{n,m}> = match at least C<n> times, but not more than C<m> |
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322 | times. |
323 | ||
cb49b31f RB |
324 | =item * |
325 | ||
326 | C<a{n,}> = match at least C<n> or more times | |
327 | ||
328 | =item * | |
47f9c88b | 329 | |
cb49b31f | 330 | C<a{n}> = match exactly C<n> times |
47f9c88b GS |
331 | |
332 | =back | |
333 | ||
334 | Here are some examples: | |
335 | ||
336 | /[a-z]+\s+\d*/; # match a lowercase word, at least some space, and | |
337 | # any number of digits | |
338 | /(\w+)\s+\1/; # match doubled words of arbitrary length | |
339 | $year =~ /\d{2,4}/; # make sure year is at least 2 but not more | |
340 | # than 4 digits | |
341 | $year =~ /\d{4}|\d{2}/; # better match; throw out 3 digit dates | |
342 | ||
343 | These quantifiers will try to match as much of the string as possible, | |
6425a278 | 344 | while still allowing the regex to match. So we have |
47f9c88b | 345 | |
6425a278 | 346 | $x = 'the cat in the hat'; |
47f9c88b GS |
347 | $x =~ /^(.*)(at)(.*)$/; # matches, |
348 | # $1 = 'the cat in the h' | |
349 | # $2 = 'at' | |
350 | # $3 = '' (0 matches) | |
351 | ||
352 | The first quantifier C<.*> grabs as much of the string as possible | |
6425a278 | 353 | while still having the regex match. The second quantifier C<.*> has |
47f9c88b GS |
354 | no string left to it, so it matches 0 times. |
355 | ||
356 | =head2 More matching | |
357 | ||
358 | There are a few more things you might want to know about matching | |
359 | operators. In the code | |
360 | ||
361 | $pattern = 'Seuss'; | |
362 | while (<>) { | |
363 | print if /$pattern/; | |
364 | } | |
365 | ||
366 | perl has to re-evaluate C<$pattern> each time through the loop. If | |
367 | C<$pattern> won't be changing, use the C<//o> modifier, to only | |
368 | perform variable substitutions once. If you don't want any | |
369 | substitutions at all, use the special delimiter C<m''>: | |
370 | ||
371 | $pattern = 'Seuss'; | |
372 | m'$pattern'; # matches '$pattern', not 'Seuss' | |
373 | ||
374 | The global modifier C<//g> allows the matching operator to match | |
375 | within a string as many times as possible. In scalar context, | |
376 | successive matches against a string will have C<//g> jump from match | |
377 | to match, keeping track of position in the string as it goes along. | |
378 | You can get or set the position with the C<pos()> function. | |
379 | For example, | |
380 | ||
381 | $x = "cat dog house"; # 3 words | |
382 | while ($x =~ /(\w+)/g) { | |
383 | print "Word is $1, ends at position ", pos $x, "\n"; | |
384 | } | |
385 | ||
386 | prints | |
387 | ||
388 | Word is cat, ends at position 3 | |
389 | Word is dog, ends at position 7 | |
390 | Word is house, ends at position 13 | |
391 | ||
392 | A failed match or changing the target string resets the position. If | |
393 | you don't want the position reset after failure to match, add the | |
6425a278 | 394 | C<//c>, as in C</regex/gc>. |
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395 | |
396 | In list context, C<//g> returns a list of matched groupings, or if | |
6425a278 | 397 | there are no groupings, a list of matches to the whole regex. So |
47f9c88b GS |
398 | |
399 | @words = ($x =~ /(\w+)/g); # matches, | |
400 | # $word[0] = 'cat' | |
401 | # $word[1] = 'dog' | |
402 | # $word[2] = 'house' | |
403 | ||
404 | =head2 Search and replace | |
405 | ||
6425a278 | 406 | Search and replace is performed using C<s/regex/replacement/modifiers>. |
47f9c88b | 407 | The C<replacement> is a Perl double quoted string that replaces in the |
6425a278 | 408 | string whatever is matched with the C<regex>. The operator C<=~> is |
47f9c88b GS |
409 | also used here to associate a string with C<s///>. If matching |
410 | against C<$_>, the S<C<$_ =~> > can be dropped. If there is a match, | |
411 | C<s///> returns the number of substitutions made, otherwise it returns | |
412 | false. Here are a few examples: | |
413 | ||
414 | $x = "Time to feed the cat!"; | |
415 | $x =~ s/cat/hacker/; # $x contains "Time to feed the hacker!" | |
416 | $y = "'quoted words'"; | |
417 | $y =~ s/^'(.*)'$/$1/; # strip single quotes, | |
418 | # $y contains "quoted words" | |
419 | ||
420 | With the C<s///> operator, the matched variables C<$1>, C<$2>, etc. | |
421 | are immediately available for use in the replacement expression. With | |
422 | the global modifier, C<s///g> will search and replace all occurrences | |
6425a278 | 423 | of the regex in the string: |
47f9c88b GS |
424 | |
425 | $x = "I batted 4 for 4"; | |
426 | $x =~ s/4/four/; # $x contains "I batted four for 4" | |
427 | $x = "I batted 4 for 4"; | |
428 | $x =~ s/4/four/g; # $x contains "I batted four for four" | |
429 | ||
430 | The evaluation modifier C<s///e> wraps an C<eval{...}> around the | |
431 | replacement string and the evaluated result is substituted for the | |
6425a278 | 432 | matched substring. Some examples: |
47f9c88b | 433 | |
6425a278 GS |
434 | # reverse all the words in a string |
435 | $x = "the cat in the hat"; | |
436 | $x =~ s/(\w+)/reverse $1/ge; # $x contains "eht tac ni eht tah" | |
47f9c88b | 437 | |
6425a278 GS |
438 | # convert percentage to decimal |
439 | $x = "A 39% hit rate"; | |
440 | $x =~ s!(\d+)%!$1/100!e; # $x contains "A 0.39 hit rate" | |
47f9c88b | 441 | |
6425a278 GS |
442 | The last example shows that C<s///> can use other delimiters, such as |
443 | C<s!!!> and C<s{}{}>, and even C<s{}//>. If single quotes are used | |
444 | C<s'''>, then the regex and replacement are treated as single quoted | |
445 | strings. | |
47f9c88b GS |
446 | |
447 | =head2 The split operator | |
448 | ||
6425a278 GS |
449 | C<split /regex/, string> splits C<string> into a list of substrings |
450 | and returns that list. The regex determines the character sequence | |
47f9c88b GS |
451 | that C<string> is split with respect to. For example, to split a |
452 | string into words, use | |
453 | ||
454 | $x = "Calvin and Hobbes"; | |
6425a278 GS |
455 | @word = split /\s+/, $x; # $word[0] = 'Calvin' |
456 | # $word[1] = 'and' | |
457 | # $word[2] = 'Hobbes' | |
458 | ||
459 | To extract a comma-delimited list of numbers, use | |
47f9c88b | 460 | |
6425a278 GS |
461 | $x = "1.618,2.718, 3.142"; |
462 | @const = split /,\s*/, $x; # $const[0] = '1.618' | |
463 | # $const[1] = '2.718' | |
464 | # $const[2] = '3.142' | |
465 | ||
466 | If the empty regex C<//> is used, the string is split into individual | |
467 | characters. If the regex has groupings, then list produced contains | |
47f9c88b GS |
468 | the matched substrings from the groupings as well: |
469 | ||
470 | $x = "/usr/bin"; | |
471 | @parts = split m!(/)!, $x; # $parts[0] = '' | |
472 | # $parts[1] = '/' | |
473 | # $parts[2] = 'usr' | |
474 | # $parts[3] = '/' | |
475 | # $parts[4] = 'bin' | |
476 | ||
6425a278 | 477 | Since the first character of $x matched the regex, C<split> prepended |
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478 | an empty initial element to the list. |
479 | ||
480 | =head1 BUGS | |
481 | ||
482 | None. | |
483 | ||
484 | =head1 SEE ALSO | |
485 | ||
486 | This is just a quick start guide. For a more in-depth tutorial on | |
6425a278 | 487 | regexes, see L<perlretut> and for the reference page, see L<perlre>. |
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488 | |
489 | =head1 AUTHOR AND COPYRIGHT | |
490 | ||
491 | Copyright (c) 2000 Mark Kvale | |
492 | All rights reserved. | |
493 | ||
494 | This document may be distributed under the same terms as Perl itself. | |
495 | ||
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496 | =head2 Acknowledgments |
497 | ||
498 | The author would like to thank Mark-Jason Dominus, Tom Christiansen, | |
499 | Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful | |
500 | comments. | |
501 | ||
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502 | =cut |
503 |