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1 | =head1 NAME |
2 | |
3 | perlre - Perl regular expressions |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | For a description of how to use regular expressions in matching |
8 | operations, see C<m//> and C<s///> in L<perlop>. The matching operations can |
9 | have various modifiers, some of which relate to the interpretation of |
10 | the regular expression inside. These are: |
11 | |
12 | i Do case-insensitive pattern matching. |
13 | m Treat string as multiple lines. |
14 | s Treat string as single line. |
15 | x Use extended regular expressions. |
16 | |
17 | These are usually written as "the C</x> modifier", even though the delimiter |
18 | in question might not actually be a slash. In fact, any of these |
19 | modifiers may also be embedded within the regular expression itself using |
20 | the new C<(?...)> construct. See below. |
21 | |
22 | The C</x> modifier itself needs a little more explanation. It tells the |
23 | regular expression parser to ignore whitespace that is not backslashed |
24 | or within a character class. You can use this to break up your regular |
25 | expression into (slightly) more readable parts. Together with the |
26 | capability of embedding comments described later, this goes a long |
27 | way towards making Perl 5 a readable language. See the C comment |
28 | deletion code in L<perlop>. |
29 | |
30 | =head2 Regular Expressions |
31 | |
32 | The patterns used in pattern matching are regular expressions such as |
33 | those supplied in the Version 8 regexp routines. (In fact, the |
34 | routines are derived (distantly) from Henry Spencer's freely |
35 | redistributable reimplementation of the V8 routines.) |
36 | See L<Version 8 Regular Expressions> for details. |
37 | |
38 | In particular the following metacharacters have their standard I<egrep>-ish |
39 | meanings: |
40 | |
41 | \ Quote the next metacharacter |
42 | ^ Match the beginning of the line |
43 | . Match any character (except newline) |
44 | $ Match the end of the line |
45 | | Alternation |
46 | () Grouping |
47 | [] Character class |
48 | |
49 | By default, the "^" character is guaranteed to match only at the |
50 | beginning of the string, the "$" character only at the end (or before the |
51 | newline at the end) and Perl does certain optimizations with the |
52 | assumption that the string contains only one line. Embedded newlines |
53 | will not be matched by "^" or "$". You may, however, wish to treat a |
54 | string as a multi-line buffer, such that the "^" will match after any |
55 | newline within the string, and "$" will match before any newline. At the |
56 | cost of a little more overhead, you can do this by using the /m modifier |
57 | on the pattern match operator. (Older programs did this by setting C<$*>, |
58 | but this practice is deprecated in Perl 5.) |
59 | |
60 | To facilitate multi-line substitutions, the "." character never matches a |
61 | newline unless you use the C</s> modifier, which tells Perl to pretend |
62 | the string is a single line--even if it isn't. The C</s> modifier also |
63 | overrides the setting of C<$*>, in case you have some (badly behaved) older |
64 | code that sets it in another module. |
65 | |
66 | The following standard quantifiers are recognized: |
67 | |
68 | * Match 0 or more times |
69 | + Match 1 or more times |
70 | ? Match 1 or 0 times |
71 | {n} Match exactly n times |
72 | {n,} Match at least n times |
73 | {n,m} Match at least n but not more than m times |
74 | |
75 | (If a curly bracket occurs in any other context, it is treated |
76 | as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+" |
77 | modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. There is no limit to the |
78 | size of n or m, but large numbers will chew up more memory. |
79 | |
80 | By default, a quantified subpattern is "greedy", that is, it will match as |
81 | many times as possible without causing the rest pattern not to match. The |
82 | standard quantifiers are all "greedy", in that they match as many |
83 | occurrences as possible (given a particular starting location) without |
84 | causing the pattern to fail. If you want it to match the minimum number |
85 | of times possible, follow the quantifier with a "?" after any of them. |
86 | Note that the meanings don't change, just the "gravity": |
87 | |
88 | *? Match 0 or more times |
89 | +? Match 1 or more times |
90 | ?? Match 0 or 1 time |
91 | {n}? Match exactly n times |
92 | {n,}? Match at least n times |
93 | {n,m}? Match at least n but not more than m times |
94 | |
95 | Since patterns are processed as double quoted strings, the following |
96 | also work: |
97 | |
98 | \t tab |
99 | \n newline |
100 | \r return |
101 | \f form feed |
102 | \v vertical tab, whatever that is |
103 | \a alarm (bell) |
104 | \e escape |
105 | \033 octal char |
106 | \x1b hex char |
107 | \c[ control char |
108 | \l lowercase next char |
109 | \u uppercase next char |
110 | \L lowercase till \E |
111 | \U uppercase till \E |
112 | \E end case modification |
113 | \Q quote regexp metacharacters till \E |
114 | |
115 | In addition, Perl defines the following: |
116 | |
117 | \w Match a "word" character (alphanumeric plus "_") |
118 | \W Match a non-word character |
119 | \s Match a whitespace character |
120 | \S Match a non-whitespace character |
121 | \d Match a digit character |
122 | \D Match a non-digit character |
123 | |
124 | Note that C<\w> matches a single alphanumeric character, not a whole |
125 | word. To match a word you'd need to say C<\w+>. You may use C<\w>, C<\W>, C<\s>, |
126 | C<\S>, C<\d> and C<\D> within character classes (though not as either end of a |
127 | range). |
128 | |
129 | Perl defines the following zero-width assertions: |
130 | |
131 | \b Match a word boundary |
132 | \B Match a non-(word boundary) |
133 | \A Match only at beginning of string |
134 | \Z Match only at end of string |
135 | \G Match only where previous m//g left off |
136 | |
137 | A word boundary (C<\b>) is defined as a spot between two characters that |
138 | has a C<\w> on one side of it and and a C<\W> on the other side of it (in |
139 | either order), counting the imaginary characters off the beginning and |
140 | end of the string as matching a C<\W>. (Within character classes C<\b> |
141 | represents backspace rather than a word boundary.) The C<\A> and C<\Z> are |
142 | just like "^" and "$" except that they won't match multiple times when the |
143 | C</m> modifier is used, while "^" and "$" will match at every internal line |
144 | boundary. |
145 | |
146 | When the bracketing construct C<( ... )> is used, \<digit> matches the |
147 | digit'th substring. (Outside of the pattern, always use "$" instead of |
148 | "\" in front of the digit. The scope of $<digit> (and C<$`>, C<$&>, and C<$')> |
149 | extends to the end of the enclosing BLOCK or eval string, or to the |
150 | next pattern match with subexpressions. |
151 | If you want to |
152 | use parentheses to delimit subpattern (e.g. a set of alternatives) without |
153 | saving it as a subpattern, follow the ( with a ?. |
154 | The \<digit> notation |
155 | sometimes works outside the current pattern, but should not be relied |
156 | upon.) You may have as many parentheses as you wish. If you have more |
157 | than 9 substrings, the variables $10, $11, ... refer to the |
158 | corresponding substring. Within the pattern, \10, \11, etc. refer back |
159 | to substrings if there have been at least that many left parens before |
160 | the backreference. Otherwise (for backward compatibilty) \10 is the |
161 | same as \010, a backspace, and \11 the same as \011, a tab. And so |
162 | on. (\1 through \9 are always backreferences.) |
163 | |
164 | C<$+> returns whatever the last bracket match matched. C<$&> returns the |
165 | entire matched string. ($0 used to return the same thing, but not any |
166 | more.) C<$`> returns everything before the matched string. C<$'> returns |
167 | everything after the matched string. Examples: |
168 | |
169 | s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words |
170 | |
171 | if (/Time: (..):(..):(..)/) { |
172 | $hours = $1; |
173 | $minutes = $2; |
174 | $seconds = $3; |
175 | } |
176 | |
177 | You will note that all backslashed metacharacters in Perl are |
178 | alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression |
179 | languages, there are no backslashed symbols that aren't alphanumeric. |
180 | So anything that looks like \\, \(, \), \<, \>, \{, or \} is always |
181 | interpreted as a literal character, not a metacharacter. This makes it |
182 | simple to quote a string that you want to use for a pattern but that |
183 | you are afraid might contain metacharacters. Simply quote all the |
184 | non-alphanumeric characters: |
185 | |
186 | $pattern =~ s/(\W)/\\$1/g; |
187 | |
188 | You can also use the built-in quotemeta() function to do this. |
189 | An even easier way to quote metacharacters right in the match operator |
190 | is to say |
191 | |
192 | /$unquoted\Q$quoted\E$unquoted/ |
193 | |
194 | Perl 5 defines a consistent extension syntax for regular expressions. |
195 | The syntax is a pair of parens with a question mark as the first thing |
196 | within the parens (this was a syntax error in Perl 4). The character |
197 | after the question mark gives the function of the extension. Several |
198 | extensions are already supported: |
199 | |
200 | =over 10 |
201 | |
202 | =item (?#text) |
203 | |
204 | A comment. The text is ignored. |
205 | |
206 | =item (?:regexp) |
207 | |
208 | This groups things like "()" but doesn't make backrefences like "()" does. So |
209 | |
210 | split(/\b(?:a|b|c)\b/) |
211 | |
212 | is like |
213 | |
214 | split(/\b(a|b|c)\b/) |
215 | |
216 | but doesn't spit out extra fields. |
217 | |
218 | =item (?=regexp) |
219 | |
220 | A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/> |
221 | matches a word followed by a tab, without including the tab in C<$&>. |
222 | |
223 | =item (?!regexp) |
224 | |
225 | A zero-width negative lookahead assertion. For example C</foo(?!bar)/> |
226 | matches any occurrence of "foo" that isn't followed by "bar". Note |
227 | however that lookahead and lookbehind are NOT the same thing. You cannot |
228 | use this for lookbehind: C</(?!foo)bar/> will not find an occurrence of |
229 | "bar" that is preceded by something which is not "foo". That's because |
230 | the C<(?!foo)> is just saying that the next thing cannot be "foo"--and |
231 | it's not, it's a "bar", so "foobar" will match. You would have to do |
232 | something like C</(?foo)...bar/> for that. We say "like" because there's |
233 | the case of your "bar" not having three characters before it. You could |
234 | cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still |
235 | easier just to say: |
236 | |
237 | if (/foo/ && $` =~ /bar$/) |
238 | |
239 | |
240 | =item (?imsx) |
241 | |
242 | One or more embedded pattern-match modifiers. This is particularly |
243 | useful for patterns that are specified in a table somewhere, some of |
244 | which want to be case sensitive, and some of which don't. The case |
245 | insensitive ones merely need to include C<(?i)> at the front of the |
246 | pattern. For example: |
247 | |
248 | $pattern = "foobar"; |
249 | if ( /$pattern/i ) |
250 | |
251 | # more flexible: |
252 | |
253 | $pattern = "(?i)foobar"; |
254 | if ( /$pattern/ ) |
255 | |
256 | =back |
257 | |
258 | The specific choice of question mark for this and the new minimal |
259 | matching construct was because 1) question mark is pretty rare in older |
260 | regular expressions, and 2) whenever you see one, you should stop |
261 | and "question" exactly what is going on. That's psychology... |
262 | |
263 | =head2 Version 8 Regular Expressions |
264 | |
265 | In case you're not familiar with the "regular" Version 8 regexp |
266 | routines, here are the pattern-matching rules not described above. |
267 | |
268 | Any single character matches itself, unless it is a I<metacharacter> |
269 | with a special meaning described here or above. You can cause |
270 | characters which normally function as metacharacters to be interpreted |
271 | literally by prefixing them with a "\" (e.g. "\." matches a ".", not any |
272 | character; "\\" matches a "\"). A series of characters matches that |
273 | series of characters in the target string, so the pattern C<blurfl> |
274 | would match "blurfl" in the target string. |
275 | |
276 | You can specify a character class, by enclosing a list of characters |
277 | in C<[]>, which will match any one of the characters in the list. If the |
278 | first character after the "[" is "^", the class matches any character not |
279 | in the list. Within a list, the "-" character is used to specify a |
280 | range, so that C<a-z> represents all the characters between "a" and "z", |
281 | inclusive. |
282 | |
283 | Characters may be specified using a metacharacter syntax much like that |
284 | used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, |
285 | "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string |
286 | of octal digits, matches the character whose ASCII value is I<nnn>. |
287 | Similarly, \xI<nn>, where I<nn> are hexidecimal digits, matches the |
288 | character whose ASCII value is I<nn>. The expression \cI<x> matches the |
289 | ASCII character control-I<x>. Finally, the "." metacharacter matches any |
290 | character except "\n" (unless you use C</s>). |
291 | |
292 | You can specify a series of alternatives for a pattern using "|" to |
293 | separate them, so that C<fee|fie|foe> will match any of "fee", "fie", |
294 | or "foe" in the target string (as would C<f(e|i|o)e>). Note that the |
295 | first alternative includes everything from the last pattern delimiter |
296 | ("(", "[", or the beginning of the pattern) up to the first "|", and |
297 | the last alternative contains everything from the last "|" to the next |
298 | pattern delimiter. For this reason, it's common practice to include |
299 | alternatives in parentheses, to minimize confusion about where they |
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300 | start and end. Note however that "|" is interpreted as a literal with |
301 | square brackets, so if you write C<[fee|fie|foe]> you're really only |
302 | matching C<[feio|]>. |
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303 | |
304 | Within a pattern, you may designate subpatterns for later reference by |
305 | enclosing them in parentheses, and you may refer back to the I<n>th |
306 | subpattern later in the pattern using the metacharacter \I<n>. |
307 | Subpatterns are numbered based on the left to right order of their |
308 | opening parenthesis. Note that a backreference matches whatever |
309 | actually matched the subpattern in the string being examined, not the |
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310 | rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will |
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311 | match "0x1234 0x4321",but not "0x1234 01234", since subpattern 1 |
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312 | actually matched "0x", even though the rule C<0|0x> could |
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313 | potentially match the leading 0 in the second number. |