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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 |
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8 | description of how to I<use> regular expressions in matching |
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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 | |
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16 | =over 4 |
17 | |
18 | =item i |
19 | |
20 | Do case-insensitive pattern matching. |
21 | |
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22 | If C<use locale> is in effect, the case map is taken from the current |
23 | locale. See L<perllocale>. |
24 | |
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25 | =item m |
26 | |
27 | Treat string as multiple lines. That is, change "^" and "$" from matching |
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28 | at only the very start or end of the string to the start or end of any |
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29 | line anywhere within the string, |
30 | |
31 | =item s |
32 | |
33 | Treat string as single line. That is, change "." to match any character |
34 | whatsoever, even a newline, which it normally would not match. |
35 | |
36 | =item x |
37 | |
38 | Extend your pattern's legibility by permitting whitespace and comments. |
39 | |
40 | =back |
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41 | |
42 | These are usually written as "the C</x> modifier", even though the delimiter |
43 | in question might not actually be a slash. In fact, any of these |
44 | modifiers may also be embedded within the regular expression itself using |
45 | the new C<(?...)> construct. See below. |
46 | |
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47 | The C</x> modifier itself needs a little more explanation. It tells |
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48 | the regular expression parser to ignore whitespace that is neither |
49 | backslashed nor within a character class. You can use this to break up |
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50 | your regular expression into (slightly) more readable parts. The C<#> |
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51 | character is also treated as a meta-character introducing a comment, |
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52 | just as in ordinary Perl code. This also means that if you want real |
53 | whitespace or C<#> characters in the pattern that you'll have to either |
54 | escape them or encode them using octal or hex escapes. Taken together, |
55 | these features go a long way towards making Perl's regular expressions |
56 | more readable. See the C comment deletion code in L<perlop>. |
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57 | |
58 | =head2 Regular Expressions |
59 | |
60 | The patterns used in pattern matching are regular expressions such as |
61 | those supplied in the Version 8 regexp routines. (In fact, the |
62 | routines are derived (distantly) from Henry Spencer's freely |
63 | redistributable reimplementation of the V8 routines.) |
64 | See L<Version 8 Regular Expressions> for details. |
65 | |
66 | In particular the following metacharacters have their standard I<egrep>-ish |
67 | meanings: |
68 | |
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69 | \ Quote the next meta-character |
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70 | ^ Match the beginning of the line |
71 | . Match any character (except newline) |
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72 | $ Match the end of the line (or before newline at the end) |
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73 | | Alternation |
74 | () Grouping |
75 | [] Character class |
76 | |
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77 | By default, the "^" character is guaranteed to match at only the |
78 | beginning of the string, the "$" character at only the end (or before the |
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79 | newline at the end) and Perl does certain optimizations with the |
80 | assumption that the string contains only one line. Embedded newlines |
81 | will not be matched by "^" or "$". You may, however, wish to treat a |
82 | string as a multi-line buffer, such that the "^" will match after any |
83 | newline within the string, and "$" will match before any newline. At the |
84 | cost of a little more overhead, you can do this by using the /m modifier |
85 | on the pattern match operator. (Older programs did this by setting C<$*>, |
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86 | but this practice is now deprecated.) |
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87 | |
88 | To facilitate multi-line substitutions, the "." character never matches a |
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89 | newline unless you use the C</s> modifier, which in effect tells Perl to pretend |
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90 | the string is a single line--even if it isn't. The C</s> modifier also |
91 | overrides the setting of C<$*>, in case you have some (badly behaved) older |
92 | code that sets it in another module. |
93 | |
94 | The following standard quantifiers are recognized: |
95 | |
96 | * Match 0 or more times |
97 | + Match 1 or more times |
98 | ? Match 1 or 0 times |
99 | {n} Match exactly n times |
100 | {n,} Match at least n times |
101 | {n,m} Match at least n but not more than m times |
102 | |
103 | (If a curly bracket occurs in any other context, it is treated |
104 | as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+" |
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105 | modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited |
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106 | to integral values less than 65536. |
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107 | |
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108 | By default, a quantified sub-pattern is "greedy", that is, it will match as |
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109 | many times as possible without causing the rest of the pattern not to match. |
110 | The standard quantifiers are all "greedy", in that they match as many |
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111 | occurrences as possible (given a particular starting location) without |
112 | causing the pattern to fail. If you want it to match the minimum number |
113 | of times possible, follow the quantifier with a "?" after any of them. |
114 | Note that the meanings don't change, just the "gravity": |
115 | |
116 | *? Match 0 or more times |
117 | +? Match 1 or more times |
118 | ?? Match 0 or 1 time |
119 | {n}? Match exactly n times |
120 | {n,}? Match at least n times |
121 | {n,m}? Match at least n but not more than m times |
122 | |
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123 | Because patterns are processed as double quoted strings, the following |
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124 | also work: |
125 | |
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126 | \t tab (HT, TAB) |
127 | \n newline (LF, NL) |
128 | \r return (CR) |
129 | \f form feed (FF) |
130 | \a alarm (bell) (BEL) |
131 | \e escape (think troff) (ESC) |
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132 | \033 octal char (think of a PDP-11) |
133 | \x1B hex char |
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134 | \c[ control char |
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135 | \l lowercase next char (think vi) |
136 | \u uppercase next char (think vi) |
137 | \L lowercase till \E (think vi) |
138 | \U uppercase till \E (think vi) |
139 | \E end case modification (think vi) |
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140 | \Q quote regexp metacharacters till \E |
141 | |
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142 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
143 | and <\U> is taken from the current locale. See L<perllocale>. |
144 | |
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145 | In addition, Perl defines the following: |
146 | |
147 | \w Match a "word" character (alphanumeric plus "_") |
148 | \W Match a non-word character |
149 | \s Match a whitespace character |
150 | \S Match a non-whitespace character |
151 | \d Match a digit character |
152 | \D Match a non-digit character |
153 | |
154 | Note that C<\w> matches a single alphanumeric character, not a whole |
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155 | word. To match a word you'd need to say C<\w+>. If C<use locale> is in |
156 | effect, the list of alphabetic characters generated by C<\w> is taken |
157 | from the current locale. See L<perllocale>. You may use C<\w>, C<\W>, |
158 | C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as |
159 | either end of a range). |
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160 | |
161 | Perl defines the following zero-width assertions: |
162 | |
163 | \b Match a word boundary |
164 | \B Match a non-(word boundary) |
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165 | \A Match at only beginning of string |
166 | \Z Match at only end of string (or before newline at the end) |
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167 | \G Match only where previous m//g left off |
168 | |
169 | A word boundary (C<\b>) is defined as a spot between two characters that |
170 | has a C<\w> on one side of it and and a C<\W> on the other side of it (in |
171 | either order), counting the imaginary characters off the beginning and |
172 | end of the string as matching a C<\W>. (Within character classes C<\b> |
173 | represents backspace rather than a word boundary.) The C<\A> and C<\Z> are |
174 | just like "^" and "$" except that they won't match multiple times when the |
175 | C</m> modifier is used, while "^" and "$" will match at every internal line |
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176 | boundary. To match the actual end of the string, not ignoring newline, |
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177 | you can use C<\Z(?!\n)>. The C<\G> assertion can be used to mix global |
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178 | matches (using C<m//g>) and non-global ones, as described in |
179 | L<perlop/"Regexp Quote-Like Operators">. |
180 | It is also useful when writing C<lex>-like scanners, when you have several |
181 | regexps which you want to match against consequent substrings of your |
182 | string, see the previous reference. |
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183 | The actual location where C<\G> will match can also be influenced |
184 | by using C<pos()> as an lvalue. See L<perlfunc/pos>. |
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185 | |
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186 | When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the |
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187 | digit'th substring. Outside of the pattern, always use "$" instead of "\" |
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188 | in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work |
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189 | outside the current pattern, this should not be relied upon. See the |
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190 | WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>) |
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191 | extends to the end of the enclosing BLOCK or eval string, or to the next |
192 | successful pattern match, whichever comes first. If you want to use |
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193 | parentheses to delimit a subpattern (e.g., a set of alternatives) without |
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194 | saving it as a subpattern, follow the ( with a ?:. |
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195 | |
196 | You may have as many parentheses as you wish. If you have more |
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197 | than 9 substrings, the variables $10, $11, ... refer to the |
198 | corresponding substring. Within the pattern, \10, \11, etc. refer back |
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199 | to substrings if there have been at least that many left parentheses before |
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200 | the backreference. Otherwise (for backward compatibility) \10 is the |
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201 | same as \010, a backspace, and \11 the same as \011, a tab. And so |
202 | on. (\1 through \9 are always backreferences.) |
203 | |
204 | C<$+> returns whatever the last bracket match matched. C<$&> returns the |
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205 | entire matched string. (C<$0> used to return the same thing, but not any |
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206 | more.) C<$`> returns everything before the matched string. C<$'> returns |
207 | everything after the matched string. Examples: |
208 | |
209 | s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words |
210 | |
211 | if (/Time: (..):(..):(..)/) { |
212 | $hours = $1; |
213 | $minutes = $2; |
214 | $seconds = $3; |
215 | } |
216 | |
217 | You will note that all backslashed metacharacters in Perl are |
218 | alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression |
219 | languages, there are no backslashed symbols that aren't alphanumeric. |
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220 | So anything that looks like \\, \(, \), \E<lt>, \E<gt>, \{, or \} is always |
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221 | interpreted as a literal character, not a meta-character. This makes it |
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222 | simple to quote a string that you want to use for a pattern but that |
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223 | you are afraid might contain metacharacters. Quote simply all the |
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224 | non-alphanumeric characters: |
225 | |
226 | $pattern =~ s/(\W)/\\$1/g; |
227 | |
228 | You can also use the built-in quotemeta() function to do this. |
229 | An even easier way to quote metacharacters right in the match operator |
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230 | is to say |
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231 | |
232 | /$unquoted\Q$quoted\E$unquoted/ |
233 | |
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234 | Perl defines a consistent extension syntax for regular expressions. |
235 | The syntax is a pair of parentheses with a question mark as the first |
236 | thing within the parentheses (this was a syntax error in older |
237 | versions of Perl). The character after the question mark gives the |
238 | function of the extension. Several extensions are already supported: |
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239 | |
240 | =over 10 |
241 | |
242 | =item (?#text) |
243 | |
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244 | A comment. The text is ignored. If the C</x> switch is used to enable |
245 | whitespace formatting, a simple C<#> will suffice. |
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246 | |
247 | =item (?:regexp) |
248 | |
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249 | This groups things like "()" but doesn't make backreferences like "()" does. So |
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250 | |
251 | split(/\b(?:a|b|c)\b/) |
252 | |
253 | is like |
254 | |
255 | split(/\b(a|b|c)\b/) |
256 | |
257 | but doesn't spit out extra fields. |
258 | |
259 | =item (?=regexp) |
260 | |
261 | A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/> |
262 | matches a word followed by a tab, without including the tab in C<$&>. |
263 | |
264 | =item (?!regexp) |
265 | |
266 | A zero-width negative lookahead assertion. For example C</foo(?!bar)/> |
267 | matches any occurrence of "foo" that isn't followed by "bar". Note |
268 | however that lookahead and lookbehind are NOT the same thing. You cannot |
269 | use this for lookbehind: C</(?!foo)bar/> will not find an occurrence of |
270 | "bar" that is preceded by something which is not "foo". That's because |
271 | the C<(?!foo)> is just saying that the next thing cannot be "foo"--and |
272 | it's not, it's a "bar", so "foobar" will match. You would have to do |
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273 | something like C</(?!foo)...bar/> for that. We say "like" because there's |
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274 | the case of your "bar" not having three characters before it. You could |
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275 | cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still |
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276 | easier just to say: |
277 | |
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278 | if (/foo/ && $` =~ /bar$/) |
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279 | |
280 | |
281 | =item (?imsx) |
282 | |
283 | One or more embedded pattern-match modifiers. This is particularly |
284 | useful for patterns that are specified in a table somewhere, some of |
285 | which want to be case sensitive, and some of which don't. The case |
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286 | insensitive ones need to include merely C<(?i)> at the front of the |
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287 | pattern. For example: |
288 | |
289 | $pattern = "foobar"; |
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290 | if ( /$pattern/i ) |
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291 | |
292 | # more flexible: |
293 | |
294 | $pattern = "(?i)foobar"; |
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295 | if ( /$pattern/ ) |
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296 | |
297 | =back |
298 | |
299 | The specific choice of question mark for this and the new minimal |
300 | matching construct was because 1) question mark is pretty rare in older |
301 | regular expressions, and 2) whenever you see one, you should stop |
302 | and "question" exactly what is going on. That's psychology... |
303 | |
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304 | =head2 Backtracking |
305 | |
306 | A fundamental feature of regular expression matching involves the notion |
307 | called I<backtracking>. which is used (when needed) by all regular |
308 | expression quantifiers, namely C<*>, C<*?>, C<+>, C<+?>, C<{n,m}>, and |
309 | C<{n,m}?>. |
310 | |
311 | For a regular expression to match, the I<entire> regular expression must |
312 | match, not just part of it. So if the beginning of a pattern containing a |
313 | quantifier succeeds in a way that causes later parts in the pattern to |
314 | fail, the matching engine backs up and recalculates the beginning |
315 | part--that's why it's called backtracking. |
316 | |
317 | Here is an example of backtracking: Let's say you want to find the |
318 | word following "foo" in the string "Food is on the foo table.": |
319 | |
320 | $_ = "Food is on the foo table."; |
321 | if ( /\b(foo)\s+(\w+)/i ) { |
322 | print "$2 follows $1.\n"; |
323 | } |
324 | |
325 | When the match runs, the first part of the regular expression (C<\b(foo)>) |
326 | finds a possible match right at the beginning of the string, and loads up |
327 | $1 with "Foo". However, as soon as the matching engine sees that there's |
328 | no whitespace following the "Foo" that it had saved in $1, it realizes its |
329 | mistake and starts over again one character after where it had had the |
330 | tentative match. This time it goes all the way until the next occurrence |
331 | of "foo". The complete regular expression matches this time, and you get |
332 | the expected output of "table follows foo." |
333 | |
334 | Sometimes minimal matching can help a lot. Imagine you'd like to match |
335 | everything between "foo" and "bar". Initially, you write something |
336 | like this: |
337 | |
338 | $_ = "The food is under the bar in the barn."; |
339 | if ( /foo(.*)bar/ ) { |
340 | print "got <$1>\n"; |
341 | } |
342 | |
343 | Which perhaps unexpectedly yields: |
344 | |
345 | got <d is under the bar in the > |
346 | |
347 | That's because C<.*> was greedy, so you get everything between the |
348 | I<first> "foo" and the I<last> "bar". In this case, it's more effective |
349 | to use minimal matching to make sure you get the text between a "foo" |
350 | and the first "bar" thereafter. |
351 | |
352 | if ( /foo(.*?)bar/ ) { print "got <$1>\n" } |
353 | got <d is under the > |
354 | |
355 | Here's another example: let's say you'd like to match a number at the end |
356 | of a string, and you also want to keep the preceding part the match. |
357 | So you write this: |
358 | |
359 | $_ = "I have 2 numbers: 53147"; |
360 | if ( /(.*)(\d*)/ ) { # Wrong! |
361 | print "Beginning is <$1>, number is <$2>.\n"; |
362 | } |
363 | |
364 | That won't work at all, because C<.*> was greedy and gobbled up the |
365 | whole string. As C<\d*> can match on an empty string the complete |
366 | regular expression matched successfully. |
367 | |
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368 | Beginning is <I have 2 numbers: 53147>, number is <>. |
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369 | |
370 | Here are some variants, most of which don't work: |
371 | |
372 | $_ = "I have 2 numbers: 53147"; |
373 | @pats = qw{ |
374 | (.*)(\d*) |
375 | (.*)(\d+) |
376 | (.*?)(\d*) |
377 | (.*?)(\d+) |
378 | (.*)(\d+)$ |
379 | (.*?)(\d+)$ |
380 | (.*)\b(\d+)$ |
381 | (.*\D)(\d+)$ |
382 | }; |
383 | |
384 | for $pat (@pats) { |
385 | printf "%-12s ", $pat; |
386 | if ( /$pat/ ) { |
387 | print "<$1> <$2>\n"; |
388 | } else { |
389 | print "FAIL\n"; |
390 | } |
391 | } |
392 | |
393 | That will print out: |
394 | |
395 | (.*)(\d*) <I have 2 numbers: 53147> <> |
396 | (.*)(\d+) <I have 2 numbers: 5314> <7> |
397 | (.*?)(\d*) <> <> |
398 | (.*?)(\d+) <I have > <2> |
399 | (.*)(\d+)$ <I have 2 numbers: 5314> <7> |
400 | (.*?)(\d+)$ <I have 2 numbers: > <53147> |
401 | (.*)\b(\d+)$ <I have 2 numbers: > <53147> |
402 | (.*\D)(\d+)$ <I have 2 numbers: > <53147> |
403 | |
404 | As you see, this can be a bit tricky. It's important to realize that a |
405 | regular expression is merely a set of assertions that gives a definition |
406 | of success. There may be 0, 1, or several different ways that the |
407 | definition might succeed against a particular string. And if there are |
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408 | multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve. |
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409 | |
410 | When using lookahead assertions and negations, this can all get even |
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411 | tricker. Imagine you'd like to find a sequence of non-digits not |
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412 | followed by "123". You might try to write that as |
413 | |
414 | $_ = "ABC123"; |
415 | if ( /^\D*(?!123)/ ) { # Wrong! |
416 | print "Yup, no 123 in $_\n"; |
417 | } |
418 | |
419 | But that isn't going to match; at least, not the way you're hoping. It |
420 | claims that there is no 123 in the string. Here's a clearer picture of |
421 | why it that pattern matches, contrary to popular expectations: |
422 | |
423 | $x = 'ABC123' ; |
424 | $y = 'ABC445' ; |
425 | |
426 | print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ; |
427 | print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ; |
428 | |
429 | print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ; |
430 | print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ; |
431 | |
432 | This prints |
433 | |
434 | 2: got ABC |
435 | 3: got AB |
436 | 4: got ABC |
437 | |
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438 | You might have expected test 3 to fail because it seems to a more |
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439 | general purpose version of test 1. The important difference between |
440 | them is that test 3 contains a quantifier (C<\D*>) and so can use |
441 | backtracking, whereas test 1 will not. What's happening is |
442 | that you've asked "Is it true that at the start of $x, following 0 or more |
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443 | non-digits, you have something that's not 123?" If the pattern matcher had |
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444 | let C<\D*> expand to "ABC", this would have caused the whole pattern to |
445 | fail. |
446 | The search engine will initially match C<\D*> with "ABC". Then it will |
447 | try to match C<(?!123> with "123" which, of course, fails. But because |
448 | a quantifier (C<\D*>) has been used in the regular expression, the |
449 | search engine can backtrack and retry the match differently |
450 | in the hope of matching the complete regular expression. |
451 | |
452 | Well now, |
453 | the pattern really, I<really> wants to succeed, so it uses the |
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454 | standard regexp back-off-and-retry and lets C<\D*> expand to just "AB" this |
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455 | time. Now there's indeed something following "AB" that is not |
456 | "123". It's in fact "C123", which suffices. |
457 | |
458 | We can deal with this by using both an assertion and a negation. We'll |
459 | say that the first part in $1 must be followed by a digit, and in fact, it |
460 | must also be followed by something that's not "123". Remember that the |
461 | lookaheads are zero-width expressions--they only look, but don't consume |
462 | any of the string in their match. So rewriting this way produces what |
463 | you'd expect; that is, case 5 will fail, but case 6 succeeds: |
464 | |
465 | print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ; |
466 | print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ; |
467 | |
468 | 6: got ABC |
469 | |
470 | In other words, the two zero-width assertions next to each other work like |
471 | they're ANDed together, just as you'd use any builtin assertions: C</^$/> |
472 | matches only if you're at the beginning of the line AND the end of the |
473 | line simultaneously. The deeper underlying truth is that juxtaposition in |
474 | regular expressions always means AND, except when you write an explicit OR |
475 | using the vertical bar. C</ab/> means match "a" AND (then) match "b", |
476 | although the attempted matches are made at different positions because "a" |
477 | is not a zero-width assertion, but a one-width assertion. |
478 | |
479 | One warning: particularly complicated regular expressions can take |
480 | exponential time to solve due to the immense number of possible ways they |
481 | can use backtracking to try match. For example this will take a very long |
482 | time to run |
483 | |
484 | /((a{0,5}){0,5}){0,5}/ |
485 | |
486 | And if you used C<*>'s instead of limiting it to 0 through 5 matches, then |
487 | it would take literally forever--or until you ran out of stack space. |
488 | |
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489 | =head2 Version 8 Regular Expressions |
490 | |
491 | In case you're not familiar with the "regular" Version 8 regexp |
492 | routines, here are the pattern-matching rules not described above. |
493 | |
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494 | Any single character matches itself, unless it is a I<meta-character> |
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495 | with a special meaning described here or above. You can cause |
496 | characters which normally function as metacharacters to be interpreted |
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497 | literally by prefixing them with a "\" (e.g., "\." matches a ".", not any |
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498 | character; "\\" matches a "\"). A series of characters matches that |
499 | series of characters in the target string, so the pattern C<blurfl> |
500 | would match "blurfl" in the target string. |
501 | |
502 | You can specify a character class, by enclosing a list of characters |
503 | in C<[]>, which will match any one of the characters in the list. If the |
504 | first character after the "[" is "^", the class matches any character not |
505 | in the list. Within a list, the "-" character is used to specify a |
506 | range, so that C<a-z> represents all the characters between "a" and "z", |
507 | inclusive. |
508 | |
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509 | Characters may be specified using a meta-character syntax much like that |
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510 | used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, |
511 | "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string |
512 | of octal digits, matches the character whose ASCII value is I<nnn>. |
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513 | Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the |
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514 | character whose ASCII value is I<nn>. The expression \cI<x> matches the |
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515 | ASCII character control-I<x>. Finally, the "." meta-character matches any |
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516 | character except "\n" (unless you use C</s>). |
517 | |
518 | You can specify a series of alternatives for a pattern using "|" to |
519 | separate them, so that C<fee|fie|foe> will match any of "fee", "fie", |
520 | or "foe" in the target string (as would C<f(e|i|o)e>). Note that the |
521 | first alternative includes everything from the last pattern delimiter |
522 | ("(", "[", or the beginning of the pattern) up to the first "|", and |
523 | the last alternative contains everything from the last "|" to the next |
524 | pattern delimiter. For this reason, it's common practice to include |
525 | alternatives in parentheses, to minimize confusion about where they |
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526 | start and end. Note however that "|" is interpreted as a literal with |
527 | square brackets, so if you write C<[fee|fie|foe]> you're really only |
528 | matching C<[feio|]>. |
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529 | |
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530 | Within a pattern, you may designate sub-patterns for later reference by |
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531 | enclosing them in parentheses, and you may refer back to the I<n>th |
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532 | sub-pattern later in the pattern using the meta-character \I<n>. |
533 | Sub-patterns are numbered based on the left to right order of their |
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534 | opening parenthesis. Note that a backreference matches whatever |
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535 | actually matched the sub-pattern in the string being examined, not the |
536 | rules for that sub-pattern. Therefore, C<(0|0x)\d*\s\1\d*> will |
537 | match "0x1234 0x4321",but not "0x1234 01234", because sub-pattern 1 |
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538 | actually matched "0x", even though the rule C<0|0x> could |
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539 | potentially match the leading 0 in the second number. |
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540 | |
541 | =head2 WARNING on \1 vs $1 |
542 | |
543 | Some people get too used to writing things like |
544 | |
545 | $pattern =~ s/(\W)/\\\1/g; |
546 | |
547 | This is grandfathered for the RHS of a substitute to avoid shocking the |
548 | B<sed> addicts, but it's a dirty habit to get into. That's because in |
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549 | PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in |
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550 | the usual double-quoted string means a control-A. The customary Unix |
551 | meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit |
552 | of doing that, you get yourself into trouble if you then add an C</e> |
553 | modifier. |
554 | |
555 | s/(\d+)/ \1 + 1 /eg; |
556 | |
557 | Or if you try to do |
558 | |
559 | s/(\d+)/\1000/; |
560 | |
561 | You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with |
562 | C<${1}000>. Basically, the operation of interpolation should not be confused |
563 | with the operation of matching a backreference. Certainly they mean two |
564 | different things on the I<left> side of the C<s///>. |