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1 | =head1 NAME |
2 | |
3 | perlre - Perl regular expressions |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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7 | This page describes the syntax of regular expressions in Perl. For a |
8 | description of how to actually I<use> regular expressions in matching |
9 | operations, plus various examples of the same, see C<m//> and C<s///> in |
10 | L<perlop>. |
11 | |
12 | The matching operations can |
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13 | have various modifiers, some of which relate to the interpretation of |
14 | the regular expression inside. These are: |
15 | |
16 | i Do case-insensitive pattern matching. |
17 | m Treat string as multiple lines. |
18 | s Treat string as single line. |
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19 | x Extend your pattern's legibility with whitespace and comments. |
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20 | |
21 | These are usually written as "the C</x> modifier", even though the delimiter |
22 | in question might not actually be a slash. In fact, any of these |
23 | modifiers may also be embedded within the regular expression itself using |
24 | the new C<(?...)> construct. See below. |
25 | |
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26 | The C</x> modifier itself needs a little more explanation. It tells |
27 | the regular expression parser to ignore whitespace that is not |
28 | backslashed or within a character class. You can use this to break up |
29 | your regular expression into (slightly) more readable parts. The C<#> |
30 | character is also treated as a metacharacter introducing a comment, |
31 | just as in ordinary Perl code. Taken together, these features go a |
32 | long way towards making Perl 5 a readable language. See the C comment |
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33 | deletion code in L<perlop>. |
34 | |
35 | =head2 Regular Expressions |
36 | |
37 | The patterns used in pattern matching are regular expressions such as |
38 | those supplied in the Version 8 regexp routines. (In fact, the |
39 | routines are derived (distantly) from Henry Spencer's freely |
40 | redistributable reimplementation of the V8 routines.) |
41 | See L<Version 8 Regular Expressions> for details. |
42 | |
43 | In particular the following metacharacters have their standard I<egrep>-ish |
44 | meanings: |
45 | |
46 | \ Quote the next metacharacter |
47 | ^ Match the beginning of the line |
48 | . Match any character (except newline) |
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49 | $ Match the end of the line (or before newline at the end) |
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50 | | Alternation |
51 | () Grouping |
52 | [] Character class |
53 | |
54 | By default, the "^" character is guaranteed to match only at the |
55 | beginning of the string, the "$" character only at the end (or before the |
56 | newline at the end) and Perl does certain optimizations with the |
57 | assumption that the string contains only one line. Embedded newlines |
58 | will not be matched by "^" or "$". You may, however, wish to treat a |
59 | string as a multi-line buffer, such that the "^" will match after any |
60 | newline within the string, and "$" will match before any newline. At the |
61 | cost of a little more overhead, you can do this by using the /m modifier |
62 | on the pattern match operator. (Older programs did this by setting C<$*>, |
63 | but this practice is deprecated in Perl 5.) |
64 | |
65 | To facilitate multi-line substitutions, the "." character never matches a |
66 | newline unless you use the C</s> modifier, which tells Perl to pretend |
67 | the string is a single line--even if it isn't. The C</s> modifier also |
68 | overrides the setting of C<$*>, in case you have some (badly behaved) older |
69 | code that sets it in another module. |
70 | |
71 | The following standard quantifiers are recognized: |
72 | |
73 | * Match 0 or more times |
74 | + Match 1 or more times |
75 | ? Match 1 or 0 times |
76 | {n} Match exactly n times |
77 | {n,} Match at least n times |
78 | {n,m} Match at least n but not more than m times |
79 | |
80 | (If a curly bracket occurs in any other context, it is treated |
81 | as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+" |
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82 | modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited |
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83 | to integral values less than 65536. |
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84 | |
85 | By default, a quantified subpattern is "greedy", that is, it will match as |
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86 | many times as possible without causing the rest of the pattern not to match. |
87 | The standard quantifiers are all "greedy", in that they match as many |
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88 | occurrences as possible (given a particular starting location) without |
89 | causing the pattern to fail. If you want it to match the minimum number |
90 | of times possible, follow the quantifier with a "?" after any of them. |
91 | Note that the meanings don't change, just the "gravity": |
92 | |
93 | *? Match 0 or more times |
94 | +? Match 1 or more times |
95 | ?? Match 0 or 1 time |
96 | {n}? Match exactly n times |
97 | {n,}? Match at least n times |
98 | {n,m}? Match at least n but not more than m times |
99 | |
100 | Since patterns are processed as double quoted strings, the following |
101 | also work: |
102 | |
103 | \t tab |
104 | \n newline |
105 | \r return |
106 | \f form feed |
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107 | \a alarm (bell) |
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108 | \e escape (think troff) |
109 | \033 octal char (think of a PDP-11) |
110 | \x1B hex char |
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111 | \c[ control char |
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112 | \l lowercase next char (think vi) |
113 | \u uppercase next char (think vi) |
114 | \L lowercase till \E (think vi) |
115 | \U uppercase till \E (think vi) |
116 | \E end case modification (think vi) |
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117 | \Q quote regexp metacharacters till \E |
118 | |
119 | In addition, Perl defines the following: |
120 | |
121 | \w Match a "word" character (alphanumeric plus "_") |
122 | \W Match a non-word character |
123 | \s Match a whitespace character |
124 | \S Match a non-whitespace character |
125 | \d Match a digit character |
126 | \D Match a non-digit character |
127 | |
128 | Note that C<\w> matches a single alphanumeric character, not a whole |
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129 | word. To match a word you'd need to say C<\w+>. You may use C<\w>, |
130 | C<\W>, C<\s>, C<\S>, C<\d> and C<\D> within character classes (though not |
131 | as either end of a range). |
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132 | |
133 | Perl defines the following zero-width assertions: |
134 | |
135 | \b Match a word boundary |
136 | \B Match a non-(word boundary) |
137 | \A Match only at beginning of string |
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138 | \Z Match only at end of string (or before newline at the end) |
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139 | \G Match only where previous m//g left off |
140 | |
141 | A word boundary (C<\b>) is defined as a spot between two characters that |
142 | has a C<\w> on one side of it and and a C<\W> on the other side of it (in |
143 | either order), counting the imaginary characters off the beginning and |
144 | end of the string as matching a C<\W>. (Within character classes C<\b> |
145 | represents backspace rather than a word boundary.) The C<\A> and C<\Z> are |
146 | just like "^" and "$" except that they won't match multiple times when the |
147 | C</m> modifier is used, while "^" and "$" will match at every internal line |
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148 | boundary. To match the actual end of the string, not ignoring newline, |
149 | you can use C<\Z(?!\n)>. |
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150 | |
151 | When the bracketing construct C<( ... )> is used, \<digit> matches the |
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152 | digit'th substring. Outside of the pattern, always use "$" instead of "\" |
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153 | in front of the digit. (While the \<digit> notation can on rare occasion work |
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154 | outside the current pattern, this should not be relied upon. See the |
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155 | WARNING below.) The scope of $<digit> (and C<$`>, C<$&>, and C<$'>) |
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156 | extends to the end of the enclosing BLOCK or eval string, or to the next |
157 | successful pattern match, whichever comes first. If you want to use |
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158 | parentheses to delimit a subpattern (e.g. a set of alternatives) without |
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159 | saving it as a subpattern, follow the ( with a ?:. |
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160 | |
161 | You may have as many parentheses as you wish. If you have more |
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162 | than 9 substrings, the variables $10, $11, ... refer to the |
163 | corresponding substring. Within the pattern, \10, \11, etc. refer back |
164 | to substrings if there have been at least that many left parens before |
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165 | the backreference. Otherwise (for backward compatibility) \10 is the |
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166 | same as \010, a backspace, and \11 the same as \011, a tab. And so |
167 | on. (\1 through \9 are always backreferences.) |
168 | |
169 | C<$+> returns whatever the last bracket match matched. C<$&> returns the |
170 | entire matched string. ($0 used to return the same thing, but not any |
171 | more.) C<$`> returns everything before the matched string. C<$'> returns |
172 | everything after the matched string. Examples: |
173 | |
174 | s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words |
175 | |
176 | if (/Time: (..):(..):(..)/) { |
177 | $hours = $1; |
178 | $minutes = $2; |
179 | $seconds = $3; |
180 | } |
181 | |
182 | You will note that all backslashed metacharacters in Perl are |
183 | alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression |
184 | languages, there are no backslashed symbols that aren't alphanumeric. |
185 | So anything that looks like \\, \(, \), \<, \>, \{, or \} is always |
186 | interpreted as a literal character, not a metacharacter. This makes it |
187 | simple to quote a string that you want to use for a pattern but that |
188 | you are afraid might contain metacharacters. Simply quote all the |
189 | non-alphanumeric characters: |
190 | |
191 | $pattern =~ s/(\W)/\\$1/g; |
192 | |
193 | You can also use the built-in quotemeta() function to do this. |
194 | An even easier way to quote metacharacters right in the match operator |
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195 | is to say |
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196 | |
197 | /$unquoted\Q$quoted\E$unquoted/ |
198 | |
199 | Perl 5 defines a consistent extension syntax for regular expressions. |
200 | The syntax is a pair of parens with a question mark as the first thing |
201 | within the parens (this was a syntax error in Perl 4). The character |
202 | after the question mark gives the function of the extension. Several |
203 | extensions are already supported: |
204 | |
205 | =over 10 |
206 | |
207 | =item (?#text) |
208 | |
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209 | A comment. The text is ignored. If the C</x> switch is used to enable |
210 | whitespace formatting, a simple C<#> will suffice. |
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211 | |
212 | =item (?:regexp) |
213 | |
214 | This groups things like "()" but doesn't make backrefences like "()" does. So |
215 | |
216 | split(/\b(?:a|b|c)\b/) |
217 | |
218 | is like |
219 | |
220 | split(/\b(a|b|c)\b/) |
221 | |
222 | but doesn't spit out extra fields. |
223 | |
224 | =item (?=regexp) |
225 | |
226 | A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/> |
227 | matches a word followed by a tab, without including the tab in C<$&>. |
228 | |
229 | =item (?!regexp) |
230 | |
231 | A zero-width negative lookahead assertion. For example C</foo(?!bar)/> |
232 | matches any occurrence of "foo" that isn't followed by "bar". Note |
233 | however that lookahead and lookbehind are NOT the same thing. You cannot |
234 | use this for lookbehind: C</(?!foo)bar/> will not find an occurrence of |
235 | "bar" that is preceded by something which is not "foo". That's because |
236 | the C<(?!foo)> is just saying that the next thing cannot be "foo"--and |
237 | it's not, it's a "bar", so "foobar" will match. You would have to do |
238 | something like C</(?foo)...bar/> for that. We say "like" because there's |
239 | the case of your "bar" not having three characters before it. You could |
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240 | cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still |
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241 | easier just to say: |
242 | |
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243 | if (/foo/ && $` =~ /bar$/) |
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244 | |
245 | |
246 | =item (?imsx) |
247 | |
248 | One or more embedded pattern-match modifiers. This is particularly |
249 | useful for patterns that are specified in a table somewhere, some of |
250 | which want to be case sensitive, and some of which don't. The case |
251 | insensitive ones merely need to include C<(?i)> at the front of the |
252 | pattern. For example: |
253 | |
254 | $pattern = "foobar"; |
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255 | if ( /$pattern/i ) |
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256 | |
257 | # more flexible: |
258 | |
259 | $pattern = "(?i)foobar"; |
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260 | if ( /$pattern/ ) |
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261 | |
262 | =back |
263 | |
264 | The specific choice of question mark for this and the new minimal |
265 | matching construct was because 1) question mark is pretty rare in older |
266 | regular expressions, and 2) whenever you see one, you should stop |
267 | and "question" exactly what is going on. That's psychology... |
268 | |
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269 | =head2 Backtracking |
270 | |
271 | A fundamental feature of regular expression matching involves the notion |
272 | called I<backtracking>. which is used (when needed) by all regular |
273 | expression quantifiers, namely C<*>, C<*?>, C<+>, C<+?>, C<{n,m}>, and |
274 | C<{n,m}?>. |
275 | |
276 | For a regular expression to match, the I<entire> regular expression must |
277 | match, not just part of it. So if the beginning of a pattern containing a |
278 | quantifier succeeds in a way that causes later parts in the pattern to |
279 | fail, the matching engine backs up and recalculates the beginning |
280 | part--that's why it's called backtracking. |
281 | |
282 | Here is an example of backtracking: Let's say you want to find the |
283 | word following "foo" in the string "Food is on the foo table.": |
284 | |
285 | $_ = "Food is on the foo table."; |
286 | if ( /\b(foo)\s+(\w+)/i ) { |
287 | print "$2 follows $1.\n"; |
288 | } |
289 | |
290 | When the match runs, the first part of the regular expression (C<\b(foo)>) |
291 | finds a possible match right at the beginning of the string, and loads up |
292 | $1 with "Foo". However, as soon as the matching engine sees that there's |
293 | no whitespace following the "Foo" that it had saved in $1, it realizes its |
294 | mistake and starts over again one character after where it had had the |
295 | tentative match. This time it goes all the way until the next occurrence |
296 | of "foo". The complete regular expression matches this time, and you get |
297 | the expected output of "table follows foo." |
298 | |
299 | Sometimes minimal matching can help a lot. Imagine you'd like to match |
300 | everything between "foo" and "bar". Initially, you write something |
301 | like this: |
302 | |
303 | $_ = "The food is under the bar in the barn."; |
304 | if ( /foo(.*)bar/ ) { |
305 | print "got <$1>\n"; |
306 | } |
307 | |
308 | Which perhaps unexpectedly yields: |
309 | |
310 | got <d is under the bar in the > |
311 | |
312 | That's because C<.*> was greedy, so you get everything between the |
313 | I<first> "foo" and the I<last> "bar". In this case, it's more effective |
314 | to use minimal matching to make sure you get the text between a "foo" |
315 | and the first "bar" thereafter. |
316 | |
317 | if ( /foo(.*?)bar/ ) { print "got <$1>\n" } |
318 | got <d is under the > |
319 | |
320 | Here's another example: let's say you'd like to match a number at the end |
321 | of a string, and you also want to keep the preceding part the match. |
322 | So you write this: |
323 | |
324 | $_ = "I have 2 numbers: 53147"; |
325 | if ( /(.*)(\d*)/ ) { # Wrong! |
326 | print "Beginning is <$1>, number is <$2>.\n"; |
327 | } |
328 | |
329 | That won't work at all, because C<.*> was greedy and gobbled up the |
330 | whole string. As C<\d*> can match on an empty string the complete |
331 | regular expression matched successfully. |
332 | |
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333 | Beginning is <I have 2 numbers: 53147>, number is <>. |
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334 | |
335 | Here are some variants, most of which don't work: |
336 | |
337 | $_ = "I have 2 numbers: 53147"; |
338 | @pats = qw{ |
339 | (.*)(\d*) |
340 | (.*)(\d+) |
341 | (.*?)(\d*) |
342 | (.*?)(\d+) |
343 | (.*)(\d+)$ |
344 | (.*?)(\d+)$ |
345 | (.*)\b(\d+)$ |
346 | (.*\D)(\d+)$ |
347 | }; |
348 | |
349 | for $pat (@pats) { |
350 | printf "%-12s ", $pat; |
351 | if ( /$pat/ ) { |
352 | print "<$1> <$2>\n"; |
353 | } else { |
354 | print "FAIL\n"; |
355 | } |
356 | } |
357 | |
358 | That will print out: |
359 | |
360 | (.*)(\d*) <I have 2 numbers: 53147> <> |
361 | (.*)(\d+) <I have 2 numbers: 5314> <7> |
362 | (.*?)(\d*) <> <> |
363 | (.*?)(\d+) <I have > <2> |
364 | (.*)(\d+)$ <I have 2 numbers: 5314> <7> |
365 | (.*?)(\d+)$ <I have 2 numbers: > <53147> |
366 | (.*)\b(\d+)$ <I have 2 numbers: > <53147> |
367 | (.*\D)(\d+)$ <I have 2 numbers: > <53147> |
368 | |
369 | As you see, this can be a bit tricky. It's important to realize that a |
370 | regular expression is merely a set of assertions that gives a definition |
371 | of success. There may be 0, 1, or several different ways that the |
372 | definition might succeed against a particular string. And if there are |
373 | multiple ways it might succeed, you need to understand backtracking in |
374 | order to know which variety of success you will achieve. |
375 | |
376 | When using lookahead assertions and negations, this can all get even |
377 | tricker. Imagine you'd like to find a sequence of nondigits not |
378 | followed by "123". You might try to write that as |
379 | |
380 | $_ = "ABC123"; |
381 | if ( /^\D*(?!123)/ ) { # Wrong! |
382 | print "Yup, no 123 in $_\n"; |
383 | } |
384 | |
385 | But that isn't going to match; at least, not the way you're hoping. It |
386 | claims that there is no 123 in the string. Here's a clearer picture of |
387 | why it that pattern matches, contrary to popular expectations: |
388 | |
389 | $x = 'ABC123' ; |
390 | $y = 'ABC445' ; |
391 | |
392 | print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ; |
393 | print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ; |
394 | |
395 | print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ; |
396 | print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ; |
397 | |
398 | This prints |
399 | |
400 | 2: got ABC |
401 | 3: got AB |
402 | 4: got ABC |
403 | |
404 | You might have expected test 3 to fail because it just seems to a more |
405 | general purpose version of test 1. The important difference between |
406 | them is that test 3 contains a quantifier (C<\D*>) and so can use |
407 | backtracking, whereas test 1 will not. What's happening is |
408 | that you've asked "Is it true that at the start of $x, following 0 or more |
409 | nondigits, you have something that's not 123?" If the pattern matcher had |
410 | let C<\D*> expand to "ABC", this would have caused the whole pattern to |
411 | fail. |
412 | The search engine will initially match C<\D*> with "ABC". Then it will |
413 | try to match C<(?!123> with "123" which, of course, fails. But because |
414 | a quantifier (C<\D*>) has been used in the regular expression, the |
415 | search engine can backtrack and retry the match differently |
416 | in the hope of matching the complete regular expression. |
417 | |
418 | Well now, |
419 | the pattern really, I<really> wants to succeed, so it uses the |
420 | standard regexp backoff-and-retry and lets C<\D*> expand to just "AB" this |
421 | time. Now there's indeed something following "AB" that is not |
422 | "123". It's in fact "C123", which suffices. |
423 | |
424 | We can deal with this by using both an assertion and a negation. We'll |
425 | say that the first part in $1 must be followed by a digit, and in fact, it |
426 | must also be followed by something that's not "123". Remember that the |
427 | lookaheads are zero-width expressions--they only look, but don't consume |
428 | any of the string in their match. So rewriting this way produces what |
429 | you'd expect; that is, case 5 will fail, but case 6 succeeds: |
430 | |
431 | print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ; |
432 | print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ; |
433 | |
434 | 6: got ABC |
435 | |
436 | In other words, the two zero-width assertions next to each other work like |
437 | they're ANDed together, just as you'd use any builtin assertions: C</^$/> |
438 | matches only if you're at the beginning of the line AND the end of the |
439 | line simultaneously. The deeper underlying truth is that juxtaposition in |
440 | regular expressions always means AND, except when you write an explicit OR |
441 | using the vertical bar. C</ab/> means match "a" AND (then) match "b", |
442 | although the attempted matches are made at different positions because "a" |
443 | is not a zero-width assertion, but a one-width assertion. |
444 | |
445 | One warning: particularly complicated regular expressions can take |
446 | exponential time to solve due to the immense number of possible ways they |
447 | can use backtracking to try match. For example this will take a very long |
448 | time to run |
449 | |
450 | /((a{0,5}){0,5}){0,5}/ |
451 | |
452 | And if you used C<*>'s instead of limiting it to 0 through 5 matches, then |
453 | it would take literally forever--or until you ran out of stack space. |
454 | |
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455 | =head2 Version 8 Regular Expressions |
456 | |
457 | In case you're not familiar with the "regular" Version 8 regexp |
458 | routines, here are the pattern-matching rules not described above. |
459 | |
460 | Any single character matches itself, unless it is a I<metacharacter> |
461 | with a special meaning described here or above. You can cause |
462 | characters which normally function as metacharacters to be interpreted |
463 | literally by prefixing them with a "\" (e.g. "\." matches a ".", not any |
464 | character; "\\" matches a "\"). A series of characters matches that |
465 | series of characters in the target string, so the pattern C<blurfl> |
466 | would match "blurfl" in the target string. |
467 | |
468 | You can specify a character class, by enclosing a list of characters |
469 | in C<[]>, which will match any one of the characters in the list. If the |
470 | first character after the "[" is "^", the class matches any character not |
471 | in the list. Within a list, the "-" character is used to specify a |
472 | range, so that C<a-z> represents all the characters between "a" and "z", |
473 | inclusive. |
474 | |
475 | Characters may be specified using a metacharacter syntax much like that |
476 | used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, |
477 | "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string |
478 | of octal digits, matches the character whose ASCII value is I<nnn>. |
479 | Similarly, \xI<nn>, where I<nn> are hexidecimal digits, matches the |
480 | character whose ASCII value is I<nn>. The expression \cI<x> matches the |
481 | ASCII character control-I<x>. Finally, the "." metacharacter matches any |
482 | character except "\n" (unless you use C</s>). |
483 | |
484 | You can specify a series of alternatives for a pattern using "|" to |
485 | separate them, so that C<fee|fie|foe> will match any of "fee", "fie", |
486 | or "foe" in the target string (as would C<f(e|i|o)e>). Note that the |
487 | first alternative includes everything from the last pattern delimiter |
488 | ("(", "[", or the beginning of the pattern) up to the first "|", and |
489 | the last alternative contains everything from the last "|" to the next |
490 | pattern delimiter. For this reason, it's common practice to include |
491 | alternatives in parentheses, to minimize confusion about where they |
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492 | start and end. Note however that "|" is interpreted as a literal with |
493 | square brackets, so if you write C<[fee|fie|foe]> you're really only |
494 | matching C<[feio|]>. |
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495 | |
496 | Within a pattern, you may designate subpatterns for later reference by |
497 | enclosing them in parentheses, and you may refer back to the I<n>th |
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498 | subpattern later in the pattern using the metacharacter \I<n>. |
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499 | Subpatterns are numbered based on the left to right order of their |
500 | opening parenthesis. Note that a backreference matches whatever |
501 | actually matched the subpattern in the string being examined, not the |
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502 | rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will |
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503 | match "0x1234 0x4321",but not "0x1234 01234", since subpattern 1 |
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504 | actually matched "0x", even though the rule C<0|0x> could |
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505 | potentially match the leading 0 in the second number. |
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506 | |
507 | =head2 WARNING on \1 vs $1 |
508 | |
509 | Some people get too used to writing things like |
510 | |
511 | $pattern =~ s/(\W)/\\\1/g; |
512 | |
513 | This is grandfathered for the RHS of a substitute to avoid shocking the |
514 | B<sed> addicts, but it's a dirty habit to get into. That's because in |
515 | PerlThink, the right-hand side of a C<s///> is a double-quoted string. C<\1> in |
516 | the usual double-quoted string means a control-A. The customary Unix |
517 | meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit |
518 | of doing that, you get yourself into trouble if you then add an C</e> |
519 | modifier. |
520 | |
521 | s/(\d+)/ \1 + 1 /eg; |
522 | |
523 | Or if you try to do |
524 | |
525 | s/(\d+)/\1000/; |
526 | |
527 | You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with |
528 | C<${1}000>. Basically, the operation of interpolation should not be confused |
529 | with the operation of matching a backreference. Certainly they mean two |
530 | different things on the I<left> side of the C<s///>. |