Commit | Line | Data |
a0d0e21e |
1 | =head1 NAME |
2 | |
3 | perlre - Perl regular expressions |
4 | |
5 | =head1 DESCRIPTION |
6 | |
cb1a09d0 |
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 |
a0d0e21e |
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. |
cb1a09d0 |
19 | x Extend your pattern's legibilty with whitespace and comments. |
a0d0e21e |
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 | |
4633a7c4 |
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 |
a0d0e21e |
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) |
49 | $ Match the end of the line |
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 "+" |
25f94b33 |
82 | modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited |
83 | to integral values less than 65536. |
a0d0e21e |
84 | |
85 | By default, a quantified subpattern is "greedy", that is, it will match as |
86 | many times as possible without causing the rest pattern not to match. The |
87 | standard quantifiers are all "greedy", in that they match as many |
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 |
107 | \v vertical tab, whatever that is |
108 | \a alarm (bell) |
cb1a09d0 |
109 | \e escape (think troff) |
110 | \033 octal char (think of a PDP-11) |
111 | \x1B hex char |
a0d0e21e |
112 | \c[ control char |
cb1a09d0 |
113 | \l lowercase next char (think vi) |
114 | \u uppercase next char (think vi) |
115 | \L lowercase till \E (think vi) |
116 | \U uppercase till \E (think vi) |
117 | \E end case modification (think vi) |
a0d0e21e |
118 | \Q quote regexp metacharacters till \E |
119 | |
120 | In addition, Perl defines the following: |
121 | |
122 | \w Match a "word" character (alphanumeric plus "_") |
123 | \W Match a non-word character |
124 | \s Match a whitespace character |
125 | \S Match a non-whitespace character |
126 | \d Match a digit character |
127 | \D Match a non-digit character |
128 | |
129 | Note that C<\w> matches a single alphanumeric character, not a whole |
cb1a09d0 |
130 | word. To match a word you'd need to say C<\w+>. You may use C<\w>, |
131 | C<\W>, C<\s>, C<\S>, C<\d> and C<\D> within character classes (though not |
132 | as either end of a range). |
a0d0e21e |
133 | |
134 | Perl defines the following zero-width assertions: |
135 | |
136 | \b Match a word boundary |
137 | \B Match a non-(word boundary) |
138 | \A Match only at beginning of string |
139 | \Z Match only at end of string |
140 | \G Match only where previous m//g left off |
141 | |
142 | A word boundary (C<\b>) is defined as a spot between two characters that |
143 | has a C<\w> on one side of it and and a C<\W> on the other side of it (in |
144 | either order), counting the imaginary characters off the beginning and |
145 | end of the string as matching a C<\W>. (Within character classes C<\b> |
146 | represents backspace rather than a word boundary.) The C<\A> and C<\Z> are |
147 | just like "^" and "$" except that they won't match multiple times when the |
148 | C</m> modifier is used, while "^" and "$" will match at every internal line |
149 | boundary. |
150 | |
151 | When the bracketing construct C<( ... )> is used, \<digit> matches the |
cb1a09d0 |
152 | digit'th substring. Outside of the pattern, always use "$" instead of "\" |
153 | in front of the digit. (The \<digit> notation can on rare occasion work |
154 | outside the current pattern, this should not be relied upon. See the |
155 | WARNING below.) The scope of $<digit> (and C<$`>, C<$&>, and C<$')> |
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 |
158 | parentheses to delimit subpattern (e.g. a set of alternatives) without |
a0d0e21e |
159 | saving it as a subpattern, follow the ( with a ?. |
cb1a09d0 |
160 | |
161 | You may have as many parentheses as you wish. If you have more |
a0d0e21e |
162 | than 9 substrings, the variables $10, $11, ... refer to the |
163 | corresponding substring. Within the pattern, \10, \11, etc. refer back |
164 | to substrings if there have been at least that many left parens before |
165 | the backreference. Otherwise (for backward compatibilty) \10 is the |
166 | same as \010, a backspace, and \11 the same as \011, a tab. And so |
167 | on. (\1 through \9 are always backreferences.) |
168 | |
169 | C<$+> 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 |
195 | is to say |
196 | |
197 | /$unquoted\Q$quoted\E$unquoted/ |
198 | |
199 | Perl 5 defines a consistent extension syntax for regular expressions. |
200 | The syntax is a pair of parens with a question mark as the first thing |
201 | within the parens (this was a syntax error in Perl 4). The character |
202 | after the question mark gives the function of the extension. Several |
203 | extensions are already supported: |
204 | |
205 | =over 10 |
206 | |
207 | =item (?#text) |
208 | |
cb1a09d0 |
209 | A comment. The text is ignored. If the C</x> switch is used to enable |
210 | whitespace formatting, a simple C<#> will suffice. |
a0d0e21e |
211 | |
212 | =item (?:regexp) |
213 | |
214 | This groups things like "()" but doesn't make backrefences like "()" does. So |
215 | |
216 | split(/\b(?:a|b|c)\b/) |
217 | |
218 | is like |
219 | |
220 | split(/\b(a|b|c)\b/) |
221 | |
222 | but doesn't spit out extra fields. |
223 | |
224 | =item (?=regexp) |
225 | |
226 | A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/> |
227 | matches a word followed by a tab, without including the tab in C<$&>. |
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 |
240 | cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still |
241 | easier just to say: |
242 | |
243 | if (/foo/ && $` =~ /bar$/) |
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 | $pattern = "foobar"; |
255 | if ( /$pattern/i ) |
256 | |
257 | # more flexible: |
258 | |
259 | $pattern = "(?i)foobar"; |
260 | if ( /$pattern/ ) |
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 | |
269 | =head2 Version 8 Regular Expressions |
270 | |
271 | In case you're not familiar with the "regular" Version 8 regexp |
272 | routines, here are the pattern-matching rules not described above. |
273 | |
274 | Any single character matches itself, unless it is a I<metacharacter> |
275 | with a special meaning described here or above. You can cause |
276 | characters which normally function as metacharacters to be interpreted |
277 | literally by prefixing them with a "\" (e.g. "\." matches a ".", not any |
278 | character; "\\" matches a "\"). A series of characters matches that |
279 | series of characters in the target string, so the pattern C<blurfl> |
280 | would match "blurfl" in the target string. |
281 | |
282 | You can specify a character class, by enclosing a list of characters |
283 | in C<[]>, which will match any one of the characters in the list. If the |
284 | first character after the "[" is "^", the class matches any character not |
285 | in the list. Within a list, the "-" character is used to specify a |
286 | range, so that C<a-z> represents all the characters between "a" and "z", |
287 | inclusive. |
288 | |
289 | Characters may be specified using a metacharacter syntax much like that |
290 | used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, |
291 | "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string |
292 | of octal digits, matches the character whose ASCII value is I<nnn>. |
293 | Similarly, \xI<nn>, where I<nn> are hexidecimal digits, matches the |
294 | character whose ASCII value is I<nn>. The expression \cI<x> matches the |
295 | ASCII character control-I<x>. Finally, the "." metacharacter matches any |
296 | character except "\n" (unless you use C</s>). |
297 | |
298 | You can specify a series of alternatives for a pattern using "|" to |
299 | separate them, so that C<fee|fie|foe> will match any of "fee", "fie", |
300 | or "foe" in the target string (as would C<f(e|i|o)e>). Note that the |
301 | first alternative includes everything from the last pattern delimiter |
302 | ("(", "[", or the beginning of the pattern) up to the first "|", and |
303 | the last alternative contains everything from the last "|" to the next |
304 | pattern delimiter. For this reason, it's common practice to include |
305 | alternatives in parentheses, to minimize confusion about where they |
748a9306 |
306 | start and end. Note however that "|" is interpreted as a literal with |
307 | square brackets, so if you write C<[fee|fie|foe]> you're really only |
308 | matching C<[feio|]>. |
a0d0e21e |
309 | |
310 | Within a pattern, you may designate subpatterns for later reference by |
311 | enclosing them in parentheses, and you may refer back to the I<n>th |
312 | subpattern later in the pattern using the metacharacter \I<n>. |
313 | Subpatterns are numbered based on the left to right order of their |
314 | opening parenthesis. Note that a backreference matches whatever |
315 | actually matched the subpattern in the string being examined, not the |
748a9306 |
316 | rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will |
a0d0e21e |
317 | match "0x1234 0x4321",but not "0x1234 01234", since subpattern 1 |
748a9306 |
318 | actually matched "0x", even though the rule C<0|0x> could |
a0d0e21e |
319 | potentially match the leading 0 in the second number. |
cb1a09d0 |
320 | |
321 | =head2 WARNING on \1 vs $1 |
322 | |
323 | Some people get too used to writing things like |
324 | |
325 | $pattern =~ s/(\W)/\\\1/g; |
326 | |
327 | This is grandfathered for the RHS of a substitute to avoid shocking the |
328 | B<sed> addicts, but it's a dirty habit to get into. That's because in |
329 | PerlThink, the right-hand side of a C<s///> is a double-quoted string. C<\1> in |
330 | the usual double-quoted string means a control-A. The customary Unix |
331 | meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit |
332 | of doing that, you get yourself into trouble if you then add an C</e> |
333 | modifier. |
334 | |
335 | s/(\d+)/ \1 + 1 /eg; |
336 | |
337 | Or if you try to do |
338 | |
339 | s/(\d+)/\1000/; |
340 | |
341 | You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with |
342 | C<${1}000>. Basically, the operation of interpolation should not be confused |
343 | with the operation of matching a backreference. Certainly they mean two |
344 | different things on the I<left> side of the C<s///>. |