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8a118206 1=head1 NAME
ea449505 2X<character class>
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3
4perlrecharclass - Perl Regular Expression Character Classes
5
6=head1 DESCRIPTION
7
8The top level documentation about Perl regular expressions
9is found in L<perlre>.
10
11This manual page discusses the syntax and use of character
6b83a163 12classes in Perl regular expressions.
8a118206 13
6b83a163 14A character class is a way of denoting a set of characters
8a118206 15in such a way that one character of the set is matched.
6b83a163 16It's important to remember that: matching a character class
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17consumes exactly one character in the source string. (The source
18string is the string the regular expression is matched against.)
19
20There are three types of character classes in Perl regular
6b83a163 21expressions: the dot, backslash sequences, and the form enclosed in square
ea449505 22brackets. Keep in mind, though, that often the term "character class" is used
6b83a163 23to mean just the bracketed form. Certainly, most Perl documentation does that.
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24
25=head2 The dot
26
27The dot (or period), C<.> is probably the most used, and certainly
28the most well-known character class. By default, a dot matches any
5db9882c 29character, except for the newline. That default can be changed to
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30add matching the newline by using the I<single line> modifier: either
31for the entire regular expression with the C</s> modifier, or
d66e1f56 32locally with C<(?s)>. (The C<L</\N>> backslash sequence, described
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33below, matches any character except newline without regard to the
34I<single line> modifier.)
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35
36Here are some examples:
37
38 "a" =~ /./ # Match
39 "." =~ /./ # Match
40 "" =~ /./ # No match (dot has to match a character)
41 "\n" =~ /./ # No match (dot does not match a newline)
42 "\n" =~ /./s # Match (global 'single line' modifier)
43 "\n" =~ /(?s:.)/ # Match (local 'single line' modifier)
44 "ab" =~ /^.$/ # No match (dot matches one character)
45
6b83a163 46=head2 Backslash sequences
82206b5e 47X<\w> X<\W> X<\s> X<\S> X<\d> X<\D> X<\p> X<\P>
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48X<\N> X<\v> X<\V> X<\h> X<\H>
49X<word> X<whitespace>
8a118206 50
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51A backslash sequence is a sequence of characters, the first one of which is a
52backslash. Perl ascribes special meaning to many such sequences, and some of
53these are character classes. That is, they match a single character each,
54provided that the character belongs to the specific set of characters defined
55by the sequence.
8a118206 56
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57Here's a list of the backslash sequences that are character classes. They
58are discussed in more detail below. (For the backslash sequences that aren't
59character classes, see L<perlrebackslash>.)
8a118206 60
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61 \d Match a decimal digit character.
62 \D Match a non-decimal-digit character.
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63 \w Match a "word" character.
64 \W Match a non-"word" character.
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65 \s Match a whitespace character.
66 \S Match a non-whitespace character.
67 \h Match a horizontal whitespace character.
68 \H Match a character that isn't horizontal whitespace.
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69 \v Match a vertical whitespace character.
70 \V Match a character that isn't vertical whitespace.
4e5e0888 71 \N Match a character that isn't a newline.
6b83a163 72 \pP, \p{Prop} Match a character that has the given Unicode property.
6c5a041f 73 \PP, \P{Prop} Match a character that doesn't have the Unicode property
8a118206 74
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75=head3 \N
76
2171640d 77C<\N>, available starting in v5.12, like the dot, matches any
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78character that is not a newline. The difference is that C<\N> is not influenced
79by the I<single line> regular expression modifier (see L</The dot> above). Note
80that the form C<\N{...}> may mean something completely different. When the
81C<{...}> is a L<quantifier|perlre/Quantifiers>, it means to match a non-newline
82character that many times. For example, C<\N{3}> means to match 3
83non-newlines; C<\N{5,}> means to match 5 or more non-newlines. But if C<{...}>
84is not a legal quantifier, it is presumed to be a named character. See
85L<charnames> for those. For example, none of C<\N{COLON}>, C<\N{4F}>, and
86C<\N{F4}> contain legal quantifiers, so Perl will try to find characters whose
87names are respectively C<COLON>, C<4F>, and C<F4>.
88
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89=head3 Digits
90
b6538e4f 91C<\d> matches a single character considered to be a decimal I<digit>.
5db9882c 92If the C</a> regular expression modifier is in effect, it matches [0-9].
582da942 93Otherwise, it
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94matches anything that is matched by C<\p{Digit}>, which includes [0-9].
95(An unlikely possible exception is that under locale matching rules, the
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96current locale might not have C<[0-9]> matched by C<\d>, and/or might match
97other characters whose code point is less than 256. The only such locale
98definitions that are legal would be to match C<[0-9]> plus another set of
9910 consecutive digit characters; anything else would be in violation of
100the C language standard, but Perl doesn't currently assume anything in
101regard to this.)
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102
103What this means is that unless the C</a> modifier is in effect C<\d> not
104only matches the digits '0' - '9', but also Arabic, Devanagari, and
105digits from other languages. This may cause some confusion, and some
106security issues.
107
108Some digits that C<\d> matches look like some of the [0-9] ones, but
109have different values. For example, BENGALI DIGIT FOUR (U+09EA) looks
110very much like an ASCII DIGIT EIGHT (U+0038). An application that
111is expecting only the ASCII digits might be misled, or if the match is
112C<\d+>, the matched string might contain a mixture of digits from
113different writing systems that look like they signify a number different
67592e11 114than they actually do. L<Unicode::UCD/num()> can
e397bccf 115be used to safely
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116calculate the value, returning C<undef> if the input string contains
117such a mixture.
118
119What C<\p{Digit}> means (and hence C<\d> except under the C</a>
120modifier) is C<\p{General_Category=Decimal_Number}>, or synonymously,
121C<\p{General_Category=Digit}>. Starting with Unicode version 4.1, this
122is the same set of characters matched by C<\p{Numeric_Type=Decimal}>.
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123But Unicode also has a different property with a similar name,
124C<\p{Numeric_Type=Digit}>, which matches a completely different set of
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125characters. These characters are things such as C<CIRCLED DIGIT ONE>
126or subscripts, or are from writing systems that lack all ten digits.
6b83a163 127
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128The design intent is for C<\d> to exactly match the set of characters
129that can safely be used with "normal" big-endian positional decimal
130syntax, where, for example 123 means one 'hundred', plus two 'tens',
131plus three 'ones'. This positional notation does not necessarily apply
132to characters that match the other type of "digit",
133C<\p{Numeric_Type=Digit}>, and so C<\d> doesn't match them.
6b83a163 134
e2cfb18c 135The Tamil digits (U+0BE6 - U+0BEF) can also legally be
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136used in old-style Tamil numbers in which they would appear no more than
137one in a row, separated by characters that mean "times 10", "times 100",
138etc. (See L<http://www.unicode.org/notes/tn21>.)
8a118206 139
b6538e4f 140Any character not matched by C<\d> is matched by C<\D>.
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141
142=head3 Word characters
143
ea449505 144A C<\w> matches a single alphanumeric character (an alphabetic character, or a
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145decimal digit); or a connecting punctuation character, such as an
146underscore ("_"); or a "mark" character (like some sort of accent) that
147attaches to one of those. It does not match a whole word. To match a
148whole word, use C<\w+>. This isn't the same thing as matching an
149English word, but in the ASCII range it is the same as a string of
150Perl-identifier characters.
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151
152=over
153
154=item If the C</a> modifier is in effect ...
155
156C<\w> matches the 63 characters [a-zA-Z0-9_].
157
158=item otherwise ...
159
160=over
161
162=item For code points above 255 ...
163
164C<\w> matches the same as C<\p{Word}> matches in this range. That is,
165it matches Thai letters, Greek letters, etc. This includes connector
d35dd6c6 166punctuation (like the underscore) which connect two words together, or
b6538e4f 167diacritics, such as a C<COMBINING TILDE> and the modifier letters, which
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168are generally used to add auxiliary markings to letters.
169
170=item For code points below 256 ...
171
172=over
173
174=item if locale rules are in effect ...
175
176C<\w> matches the platform's native underscore character plus whatever
177the locale considers to be alphanumeric.
178
4b9734bf 179=item if Unicode rules are in effect ...
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180
181C<\w> matches exactly what C<\p{Word}> matches.
182
183=item otherwise ...
184
185C<\w> matches [a-zA-Z0-9_].
186
187=back
188
189=back
190
191=back
192
193Which rules apply are determined as described in L<perlre/Which character set modifier is in effect?>.
8a118206 194
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195There are a number of security issues with the full Unicode list of word
196characters. See L<http://unicode.org/reports/tr36>.
197
198Also, for a somewhat finer-grained set of characters that are in programming
199language identifiers beyond the ASCII range, you may wish to instead use the
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200more customized L</Unicode Properties>, C<\p{ID_Start}>,
201C<\p{ID_Continue}>, C<\p{XID_Start}>, and C<\p{XID_Continue}>. See
202L<http://unicode.org/reports/tr31>.
6b83a163 203
b6538e4f 204Any character not matched by C<\w> is matched by C<\W>.
8a118206 205
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206=head3 Whitespace
207
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208C<\s> matches any single character considered whitespace.
209
210=over
211
212=item If the C</a> modifier is in effect ...
213
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214In all Perl versions, C<\s> matches the 5 characters [\t\n\f\r ]; that
215is, the horizontal tab,
216the newline, the form feed, the carriage return, and the space.
217Starting in Perl v5.18, experimentally, it also matches the vertical tab, C<\cK>.
218See note C<[1]> below for a discussion of this.
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219
220=item otherwise ...
221
222=over
223
224=item For code points above 255 ...
225
226C<\s> matches exactly the code points above 255 shown with an "s" column
227in the table below.
228
229=item For code points below 256 ...
230
231=over
232
233=item if locale rules are in effect ...
234
d28d8023 235C<\s> matches whatever the locale considers to be whitespace.
82206b5e 236
4b9734bf 237=item if Unicode rules are in effect ...
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238
239C<\s> matches exactly the characters shown with an "s" column in the
240table below.
241
242=item otherwise ...
243
2941e8b2 244C<\s> matches [\t\n\f\r ] and, starting, experimentally in Perl
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245v5.18, the vertical tab, C<\cK>.
246(See note C<[1]> below for a discussion of this.)
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247Note that this list doesn't include the non-breaking space.
248
249=back
250
251=back
252
253=back
254
255Which rules apply are determined as described in L<perlre/Which character set modifier is in effect?>.
8a118206 256
b6538e4f 257Any character not matched by C<\s> is matched by C<\S>.
8a118206 258
b6538e4f 259C<\h> matches any character considered horizontal whitespace;
8129baca 260this includes the platform's space and tab characters and several others
b6538e4f 261listed in the table below. C<\H> matches any character
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262not considered horizontal whitespace. They use the platform's native
263character set, and do not consider any locale that may otherwise be in
264use.
ea449505 265
b6538e4f 266C<\v> matches any character considered vertical whitespace;
8129baca 267this includes the platform's carriage return and line feed characters (newline)
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268plus several other characters, all listed in the table below.
269C<\V> matches any character not considered vertical whitespace.
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270They use the platform's native character set, and do not consider any
271locale that may otherwise be in use.
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272
273C<\R> matches anything that can be considered a newline under Unicode
274rules. It's not a character class, as it can match a multi-character
275sequence. Therefore, it cannot be used inside a bracketed character
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276class; use C<\v> instead (vertical whitespace). It uses the platform's
277native character set, and does not consider any locale that may
278otherwise be in use.
ea449505 279Details are discussed in L<perlrebackslash>.
8a118206 280
82206b5e 281Note that unlike C<\s> (and C<\d> and C<\w>), C<\h> and C<\v> always match
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282the same characters, without regard to other factors, such as the active
283locale or whether the source string is in UTF-8 format.
8a118206 284
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285One might think that C<\s> is equivalent to C<[\h\v]>. This is indeed true
286starting in Perl v5.18, but prior to that, the sole difference was that the
287vertical tab (C<"\cK">) was not matched by C<\s>.
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288
289The following table is a complete listing of characters matched by
a9c9e371 290C<\s>, C<\h> and C<\v> as of Unicode 6.3.
8a118206 291
582da942 292The first column gives the Unicode code point of the character (in hex format),
8a118206 293the second column gives the (Unicode) name. The third column indicates
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294by which class(es) the character is matched (assuming no locale is in
295effect that changes the C<\s> matching).
8a118206 296
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297 0x0009 CHARACTER TABULATION h s
298 0x000a LINE FEED (LF) vs
d28d8023 299 0x000b LINE TABULATION vs [1]
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300 0x000c FORM FEED (FF) vs
301 0x000d CARRIAGE RETURN (CR) vs
302 0x0020 SPACE h s
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303 0x0085 NEXT LINE (NEL) vs [2]
304 0x00a0 NO-BREAK SPACE h s [2]
fc28d2a3 305 0x1680 OGHAM SPACE MARK h s
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306 0x2000 EN QUAD h s
307 0x2001 EM QUAD h s
308 0x2002 EN SPACE h s
309 0x2003 EM SPACE h s
310 0x2004 THREE-PER-EM SPACE h s
311 0x2005 FOUR-PER-EM SPACE h s
312 0x2006 SIX-PER-EM SPACE h s
313 0x2007 FIGURE SPACE h s
314 0x2008 PUNCTUATION SPACE h s
315 0x2009 THIN SPACE h s
316 0x200a HAIR SPACE h s
317 0x2028 LINE SEPARATOR vs
318 0x2029 PARAGRAPH SEPARATOR vs
319 0x202f NARROW NO-BREAK SPACE h s
320 0x205f MEDIUM MATHEMATICAL SPACE h s
321 0x3000 IDEOGRAPHIC SPACE h s
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322
323=over 4
324
325=item [1]
326
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327Prior to Perl v5.18, C<\s> did not match the vertical tab. The change
328in v5.18 is considered an experiment, which means it could be backed out
a04e6aad 329in v5.22 if experience indicates that it breaks too much
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330existing code. If this change adversely affects you, send email to
331C<perlbug@perl.org>; if it affects you positively, email
332C<perlthanks@perl.org>. In the meantime, C<[^\S\cK]> (obscurely)
333matches what C<\s> traditionally did.
334
335=item [2]
336
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337NEXT LINE and NO-BREAK SPACE may or may not match C<\s> depending
338on the rules in effect. See
339L<the beginning of this section|/Whitespace>.
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340
341=back
342
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343=head3 Unicode Properties
344
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345C<\pP> and C<\p{Prop}> are character classes to match characters that fit given
346Unicode properties. One letter property names can be used in the C<\pP> form,
347with the property name following the C<\p>, otherwise, braces are required.
348When using braces, there is a single form, which is just the property name
349enclosed in the braces, and a compound form which looks like C<\p{name=value}>,
b6538e4f 350which means to match if the property "name" for the character has that particular
c1c4ae3a 351"value".
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352For instance, a match for a number can be written as C</\pN/> or as
353C</\p{Number}/>, or as C</\p{Number=True}/>.
354Lowercase letters are matched by the property I<Lowercase_Letter> which
e2cfb18c 355has the short form I<Ll>. They need the braces, so are written as C</\p{Ll}/> or
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356C</\p{Lowercase_Letter}/>, or C</\p{General_Category=Lowercase_Letter}/>
357(the underscores are optional).
358C</\pLl/> is valid, but means something different.
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359It matches a two character string: a letter (Unicode property C<\pL>),
360followed by a lowercase C<l>.
361
bc943be5 362If locale rules are not in effect, the use of
82206b5e 363a Unicode property will force the regular expression into using Unicode
bc943be5 364rules, if it isn't already.
82206b5e 365
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366Note that almost all properties are immune to case-insensitive matching.
367That is, adding a C</i> regular expression modifier does not change what
82206b5e 368they match. There are two sets that are affected. The first set is
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369C<Uppercase_Letter>,
370C<Lowercase_Letter>,
371and C<Titlecase_Letter>,
372all of which match C<Cased_Letter> under C</i> matching.
b6538e4f 373The second set is
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374C<Uppercase>,
375C<Lowercase>,
376and C<Titlecase>,
377all of which match C<Cased> under C</i> matching.
378(The difference between these sets is that some things, such as Roman
e2cfb18c 379numerals, come in both upper and lower case, so they are C<Cased>, but
b6538e4f 380aren't considered to be letters, so they aren't C<Cased_Letter>s. They're
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381actually C<Letter_Number>s.)
382This set also includes its subsets C<PosixUpper> and C<PosixLower>, both
e2cfb18c 383of which under C</i> match C<PosixAlpha>.
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384
385For more details on Unicode properties, see L<perlunicode/Unicode
386Character Properties>; for a
e1b711da 387complete list of possible properties, see
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388L<perluniprops/Properties accessible through \p{} and \P{}>,
389which notes all forms that have C</i> differences.
e1b711da 390It is also possible to define your own properties. This is discussed in
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391L<perlunicode/User-Defined Character Properties>.
392
94b42e47 393Unicode properties are defined (surprise!) only on Unicode code points.
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394Starting in v5.20, when matching against C<\p> and C<\P>, Perl treats
395non-Unicode code points (those above the legal Unicode maximum of
3960x10FFFF) as if they were typical unassigned Unicode code points.
94b42e47 397
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398Prior to v5.20, Perl raised a warning and made all matches fail on
399non-Unicode code points. This could be somewhat surprising:
94b42e47 400
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401 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails on Perls < v5.20.
402 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Also fails on Perls
403 # < v5.20
404
405Even though these two matches might be thought of as complements, until
406v5.20 they were so only on Unicode code points.
94b42e47 407
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408=head4 Examples
409
410 "a" =~ /\w/ # Match, "a" is a 'word' character.
411 "7" =~ /\w/ # Match, "7" is a 'word' character as well.
412 "a" =~ /\d/ # No match, "a" isn't a digit.
413 "7" =~ /\d/ # Match, "7" is a digit.
ea449505 414 " " =~ /\s/ # Match, a space is whitespace.
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415 "a" =~ /\D/ # Match, "a" is a non-digit.
416 "7" =~ /\D/ # No match, "7" is not a non-digit.
ea449505 417 " " =~ /\S/ # No match, a space is not non-whitespace.
8a118206 418
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419 " " =~ /\h/ # Match, space is horizontal whitespace.
420 " " =~ /\v/ # No match, space is not vertical whitespace.
421 "\r" =~ /\v/ # Match, a return is vertical whitespace.
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422
423 "a" =~ /\pL/ # Match, "a" is a letter.
424 "a" =~ /\p{Lu}/ # No match, /\p{Lu}/ matches upper case letters.
425
426 "\x{0e0b}" =~ /\p{Thai}/ # Match, \x{0e0b} is the character
427 # 'THAI CHARACTER SO SO', and that's in
428 # Thai Unicode class.
ea449505 429 "a" =~ /\P{Lao}/ # Match, as "a" is not a Laotian character.
8a118206 430
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431It is worth emphasizing that C<\d>, C<\w>, etc, match single characters, not
432complete numbers or words. To match a number (that consists of digits),
433use C<\d+>; to match a word, use C<\w+>. But be aware of the security
434considerations in doing so, as mentioned above.
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435
436=head2 Bracketed Character Classes
437
438The third form of character class you can use in Perl regular expressions
6b83a163 439is the bracketed character class. In its simplest form, it lists the characters
c1c4ae3a 440that may be matched, surrounded by square brackets, like this: C<[aeiou]>.
ea449505 441This matches one of C<a>, C<e>, C<i>, C<o> or C<u>. Like the other
1f59b283 442character classes, exactly one character is matched.* To match
ea449505 443a longer string consisting of characters mentioned in the character
6b83a163 444class, follow the character class with a L<quantifier|perlre/Quantifiers>. For
b6538e4f 445instance, C<[aeiou]+> matches one or more lowercase English vowels.
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446
447Repeating a character in a character class has no
448effect; it's considered to be in the set only once.
449
450Examples:
451
452 "e" =~ /[aeiou]/ # Match, as "e" is listed in the class.
453 "p" =~ /[aeiou]/ # No match, "p" is not listed in the class.
454 "ae" =~ /^[aeiou]$/ # No match, a character class only matches
455 # a single character.
456 "ae" =~ /^[aeiou]+$/ # Match, due to the quantifier.
457
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458 -------
459
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460* There are two exceptions to a bracketed character class matching a
461single character only. Each requires special handling by Perl to make
462things work:
463
464=over
465
466=item *
467
468When the class is to match caselessly under C</i> matching rules, and a
469character that is explicitly mentioned inside the class matches a
1f59b283 470multiple-character sequence caselessly under Unicode rules, the class
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471will also match that sequence. For example, Unicode says that the
472letter C<LATIN SMALL LETTER SHARP S> should match the sequence C<ss>
473under C</i> rules. Thus,
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474
475 'ss' =~ /\A\N{LATIN SMALL LETTER SHARP S}\z/i # Matches
476 'ss' =~ /\A[aeioust\N{LATIN SMALL LETTER SHARP S}]\z/i # Matches
477
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478For this to happen, the class must not be inverted (see L</Negation>)
479and the character must be explicitly specified, and not be part of a
480multi-character range (not even as one of its endpoints). (L</Character
481Ranges> will be explained shortly.) Therefore,
9d53c457 482
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483 'ss' =~ /\A[\0-\x{ff}]\z/ui # Doesn't match
484 'ss' =~ /\A[\0-\N{LATIN SMALL LETTER SHARP S}]\z/ui # No match
485 'ss' =~ /\A[\xDF-\xDF]\z/ui # Matches on ASCII platforms, since
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486 # \XDF is LATIN SMALL LETTER SHARP S,
487 # and the range is just a single
488 # element
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489
490Note that it isn't a good idea to specify these types of ranges anyway.
491
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492=item *
493
494Some names known to C<\N{...}> refer to a sequence of multiple characters,
495instead of the usual single character. When one of these is included in
496the class, the entire sequence is matched. For example,
497
498 "\N{TAMIL LETTER KA}\N{TAMIL VOWEL SIGN AU}"
499 =~ / ^ [\N{TAMIL SYLLABLE KAU}] $ /x;
500
501matches, because C<\N{TAMIL SYLLABLE KAU}> is a named sequence
502consisting of the two characters matched against. Like the other
eb9e3b14 503instance where a bracketed class can match multiple characters, and for
8f0cd35a
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504similar reasons, the class must not be inverted, and the named sequence
505may not appear in a range, even one where it is both endpoints. If
506these happen, it is a fatal error if the character class is within an
507extended L<C<(?[...])>|/Extended Bracketed Character Classes>
508class; and only the first code point is used (with
509a C<regexp>-type warning raised) otherwise.
510
511=back
512
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513=head3 Special Characters Inside a Bracketed Character Class
514
515Most characters that are meta characters in regular expressions (that
df225385 516is, characters that carry a special meaning like C<.>, C<*>, or C<(>) lose
8a118206
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517their special meaning and can be used inside a character class without
518the need to escape them. For instance, C<[()]> matches either an opening
519parenthesis, or a closing parenthesis, and the parens inside the character
520class don't group or capture.
521
522Characters that may carry a special meaning inside a character class are:
523C<\>, C<^>, C<->, C<[> and C<]>, and are discussed below. They can be
524escaped with a backslash, although this is sometimes not needed, in which
525case the backslash may be omitted.
526
527The sequence C<\b> is special inside a bracketed character class. While
6b83a163 528outside the character class, C<\b> is an assertion indicating a point
8a118206
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529that does not have either two word characters or two non-word characters
530on either side, inside a bracketed character class, C<\b> matches a
531backspace character.
532
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533The sequences
534C<\a>,
535C<\c>,
536C<\e>,
537C<\f>,
538C<\n>,
e526e8bb 539C<\N{I<NAME>}>,
765fa144 540C<\N{U+I<hex char>}>,
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541C<\r>,
542C<\t>,
543and
544C<\x>
06ee63cd 545are also special and have the same meanings as they do outside a
eb9e3b14 546bracketed character class.
df225385 547
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548Also, a backslash followed by two or three octal digits is considered an octal
549number.
df225385 550
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551A C<[> is not special inside a character class, unless it's the start of a
552POSIX character class (see L</POSIX Character Classes> below). It normally does
553not need escaping.
8a118206 554
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555A C<]> is normally either the end of a POSIX character class (see
556L</POSIX Character Classes> below), or it signals the end of the bracketed
557character class. If you want to include a C<]> in the set of characters, you
558must generally escape it.
b6538e4f 559
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560However, if the C<]> is the I<first> (or the second if the first
561character is a caret) character of a bracketed character class, it
562does not denote the end of the class (as you cannot have an empty class)
563and is considered part of the set of characters that can be matched without
564escaping.
565
566Examples:
567
568 "+" =~ /[+?*]/ # Match, "+" in a character class is not special.
090752cc 569 "\cH" =~ /[\b]/ # Match, \b inside in a character class
c1c4ae3a 570 # is equivalent to a backspace.
090752cc 571 "]" =~ /[][]/ # Match, as the character class contains
8a118206
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572 # both [ and ].
573 "[]" =~ /[[]]/ # Match, the pattern contains a character class
52f4d632 574 # containing just [, and the character class is
8a118206
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575 # followed by a ].
576
577=head3 Character Ranges
578
579It is not uncommon to want to match a range of characters. Luckily, instead
b6538e4f 580of listing all characters in the range, one may use the hyphen (C<->).
8a118206 581If inside a bracketed character class you have two characters separated
b6538e4f 582by a hyphen, it's treated as if all characters between the two were in
8a118206 583the class. For instance, C<[0-9]> matches any ASCII digit, and C<[a-m]>
e2cfb18c 584matches any lowercase letter from the first half of the ASCII alphabet.
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585
586Note that the two characters on either side of the hyphen are not
765fa144 587necessarily both letters or both digits. Any character is possible,
8a118206 588although not advisable. C<['-?]> contains a range of characters, but
b6538e4f 589most people will not know which characters that means. Furthermore,
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590such ranges may lead to portability problems if the code has to run on
591a platform that uses a different character set, such as EBCDIC.
592
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593If a hyphen in a character class cannot syntactically be part of a range, for
594instance because it is the first or the last character of the character class,
b6538e4f
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595or if it immediately follows a range, the hyphen isn't special, and so is
596considered a character to be matched literally. If you want a hyphen in
597your set of characters to be matched and its position in the class is such
598that it could be considered part of a range, you must escape that hyphen
599with a backslash.
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600
601Examples:
602
603 [a-z] # Matches a character that is a lower case ASCII letter.
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604 [a-fz] # Matches any letter between 'a' and 'f' (inclusive) or
605 # the letter 'z'.
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606 [-z] # Matches either a hyphen ('-') or the letter 'z'.
607 [a-f-m] # Matches any letter between 'a' and 'f' (inclusive), the
608 # hyphen ('-'), or the letter 'm'.
609 ['-?] # Matches any of the characters '()*+,-./0123456789:;<=>?
610 # (But not on an EBCDIC platform).
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611 [\N{APOSTROPHE}-\N{QUESTION MARK}]
612 # Matches any of the characters '()*+,-./0123456789:;<=>?
613 # even on an EBCDIC platform.
ad63362f 614 [\N{U+27}-\N{U+3F}] # Same. (U+27 is "'", and U+3F is "?")
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615
616As the final two examples above show, you can achieve portablity to
617non-ASCII platforms by using the C<\N{...}> form for the range
618endpoints. These indicate that the specified range is to be interpreted
619using Unicode values, so C<[\N{U+27}-\N{U+3F}]> means to match
620C<\N{U+27}>, C<\N{U+28}>, C<\N{U+29}>, ..., C<\N{U+3D}>, C<\N{U+3E}>,
621and C<\N{U+3F}>, whatever the native code point versions for those are.
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622These are called "Unicode" ranges. If either end is of the C<\N{...}>
623form, the range is considered Unicode. A C<regexp> warning is raised
624under C<S<"use re 'strict'">> if the other endpoint is specified
625non-portably:
626
627 [\N{U+00}-\x09] # Warning under re 'strict'; \x09 is non-portable
628 [\N{U+00}-\t] # No warning;
629
630Both of the above match the characters C<\N{U+00}> C<\N{U+01}>, ...
631C<\N{U+08}>, C<\N{U+09}>, but the C<\x09> looks like it could be a
632mistake so the warning is raised (under C<re 'strict'>) for it.
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633
634Perl also guarantees that the ranges C<A-Z>, C<a-z>, C<0-9>, and any
09e43397 635subranges of these match what an English-only speaker would expect them
c7d25594
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636to match on any platform. That is, C<[A-Z]> matches the 26 ASCII
637uppercase letters;
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638C<[a-z]> matches the 26 lowercase letters; and C<[0-9]> matches the 10
639digits. Subranges, like C<[h-k]>, match correspondingly, in this case
640just the four letters C<"h">, C<"i">, C<"j">, and C<"k">. This is the
641natural behavior on ASCII platforms where the code points (ordinal
642values) for C<"h"> through C<"k"> are consecutive integers (0x68 through
6430x6B). But special handling to achieve this may be needed on platforms
644with a non-ASCII native character set. For example, on EBCDIC
645platforms, the code point for C<"h"> is 0x88, C<"i"> is 0x89, C<"j"> is
6460x91, and C<"k"> is 0x92. Perl specially treats C<[h-k]> to exclude the
647seven code points in the gap: 0x8A through 0x90. This special handling is
648only invoked when the range is a subrange of one of the ASCII uppercase,
649lowercase, and digit ranges, AND each end of the range is expressed
650either as a literal, like C<"A">, or as a named character (C<\N{...}>,
651including the C<\N{U+...> form).
652
653EBCDIC Examples:
654
655 [i-j] # Matches either "i" or "j"
656 [i-\N{LATIN SMALL LETTER J}] # Same
657 [i-\N{U+6A}] # Same
658 [\N{U+69}-\N{U+6A}] # Same
659 [\x{89}-\x{91}] # Matches 0x89 ("i"), 0x8A .. 0x90, 0x91 ("j")
660 [i-\x{91}] # Same
661 [\x{89}-j] # Same
662 [i-J] # Matches, 0x89 ("i") .. 0xC1 ("J"); special
663 # handling doesn't apply because range is mixed
664 # case
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665
666=head3 Negation
667
668It is also possible to instead list the characters you do not want to
669match. You can do so by using a caret (C<^>) as the first character in the
b6538e4f 670character class. For instance, C<[^a-z]> matches any character that is not a
e2cfb18c
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671lowercase ASCII letter, which therefore includes more than a million
672Unicode code points. The class is said to be "negated" or "inverted".
8a118206
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673
674This syntax make the caret a special character inside a bracketed character
675class, but only if it is the first character of the class. So if you want
82206b5e 676the caret as one of the characters to match, either escape the caret or
e2cfb18c 677else don't list it first.
8a118206 678
1f59b283 679In inverted bracketed character classes, Perl ignores the Unicode rules
8f0cd35a
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680that normally say that named sequence, and certain characters should
681match a sequence of multiple characters use under caseless C</i>
682matching. Following those rules could lead to highly confusing
683situations:
1f59b283 684
582da942 685 "ss" =~ /^[^\xDF]+$/ui; # Matches!
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686
687This should match any sequences of characters that aren't C<\xDF> nor
688what C<\xDF> matches under C</i>. C<"s"> isn't C<\xDF>, but Unicode
689says that C<"ss"> is what C<\xDF> matches under C</i>. So which one
690"wins"? Do you fail the match because the string has C<ss> or accept it
582da942 691because it has an C<s> followed by another C<s>? Perl has chosen the
8f0cd35a 692latter. (See note in L</Bracketed Character Classes> above.)
1f59b283 693
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694Examples:
695
696 "e" =~ /[^aeiou]/ # No match, the 'e' is listed.
697 "x" =~ /[^aeiou]/ # Match, as 'x' isn't a lowercase vowel.
698 "^" =~ /[^^]/ # No match, matches anything that isn't a caret.
699 "^" =~ /[x^]/ # Match, caret is not special here.
700
701=head3 Backslash Sequences
702
ea449505 703You can put any backslash sequence character class (with the exception of
765fa144 704C<\N> and C<\R>) inside a bracketed character class, and it will act just
b6538e4f
TC
705as if you had put all characters matched by the backslash sequence inside the
706character class. For instance, C<[a-f\d]> matches any decimal digit, or any
6b83a163
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707of the lowercase letters between 'a' and 'f' inclusive.
708
709C<\N> within a bracketed character class must be of the forms C<\N{I<name>}>
765fa144 710or C<\N{U+I<hex char>}>, and NOT be the form that matches non-newlines,
6b83a163
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711for the same reason that a dot C<.> inside a bracketed character class loses
712its special meaning: it matches nearly anything, which generally isn't what you
713want to happen.
df225385 714
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715
716Examples:
717
718 /[\p{Thai}\d]/ # Matches a character that is either a Thai
719 # character, or a digit.
720 /[^\p{Arabic}()]/ # Matches a character that is neither an Arabic
721 # character, nor a parenthesis.
722
723Backslash sequence character classes cannot form one of the endpoints
6b83a163
KW
724of a range. Thus, you can't say:
725
726 /[\p{Thai}-\d]/ # Wrong!
8a118206 727
6b83a163 728=head3 POSIX Character Classes
ea449505 729X<character class> X<\p> X<\p{}>
ea449505
KW
730X<alpha> X<alnum> X<ascii> X<blank> X<cntrl> X<digit> X<graph>
731X<lower> X<print> X<punct> X<space> X<upper> X<word> X<xdigit>
8a118206 732
d66e1f56 733POSIX character classes have the form C<[:class:]>, where I<class> is the
6b83a163 734name, and the C<[:> and C<:]> delimiters. POSIX character classes only appear
8a118206 735I<inside> bracketed character classes, and are a convenient and descriptive
82206b5e 736way of listing a group of characters.
6b83a163
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737
738Be careful about the syntax,
8a118206
RGS
739
740 # Correct:
741 $string =~ /[[:alpha:]]/
742
743 # Incorrect (will warn):
744 $string =~ /[:alpha:]/
745
746The latter pattern would be a character class consisting of a colon,
747and the letters C<a>, C<l>, C<p> and C<h>.
d66e1f56 748
82206b5e 749POSIX character classes can be part of a larger bracketed character class.
b6538e4f 750For example,
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751
752 [01[:alpha:]%]
753
754is valid and matches '0', '1', any alphabetic character, and the percent sign.
8a118206
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755
756Perl recognizes the following POSIX character classes:
757
ea449505 758 alpha Any alphabetical character ("[A-Za-z]").
48cbae4f 759 alnum Any alphanumeric character ("[A-Za-z0-9]").
ea449505 760 ascii Any character in the ASCII character set.
ea8b8ad2 761 blank A GNU extension, equal to a space or a horizontal tab ("\t").
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762 cntrl Any control character. See Note [2] below.
763 digit Any decimal digit ("[0-9]"), equivalent to "\d".
764 graph Any printable character, excluding a space. See Note [3] below.
765 lower Any lowercase character ("[a-z]").
766 print Any printable character, including a space. See Note [4] below.
c1c4ae3a 767 punct Any graphical character excluding "word" characters. Note [5].
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768 space Any whitespace character. "\s" including the vertical tab
769 ("\cK").
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770 upper Any uppercase character ("[A-Z]").
771 word A Perl extension ("[A-Za-z0-9_]"), equivalent to "\w".
772 xdigit Any hexadecimal digit ("[0-9a-fA-F]").
773
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774Like the L<Unicode properties|/Unicode Properties>, most of the POSIX
775properties match the same regardless of whether case-insensitive (C</i>)
776matching is in effect or not. The two exceptions are C<[:upper:]> and
777C<[:lower:]>. Under C</i>, they each match the union of C<[:upper:]> and
778C<[:lower:]>.
779
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780Most POSIX character classes have two Unicode-style C<\p> property
781counterparts. (They are not official Unicode properties, but Perl extensions
782derived from official Unicode properties.) The table below shows the relation
783between POSIX character classes and these counterparts.
784
785One counterpart, in the column labelled "ASCII-range Unicode" in
b6538e4f 786the table, matches only characters in the ASCII character set.
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787
788The other counterpart, in the column labelled "Full-range Unicode", matches any
789appropriate characters in the full Unicode character set. For example,
b6538e4f 790C<\p{Alpha}> matches not just the ASCII alphabetic characters, but any
82206b5e 791character in the entire Unicode character set considered alphabetic.
582da942 792An entry in the column labelled "backslash sequence" is a (short)
5db9882c 793equivalent.
ea449505 794
cbc24f92
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795 [[:...:]] ASCII-range Full-range backslash Note
796 Unicode Unicode sequence
ea449505 797 -----------------------------------------------------
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KW
798 alpha \p{PosixAlpha} \p{XPosixAlpha}
799 alnum \p{PosixAlnum} \p{XPosixAlnum}
82206b5e 800 ascii \p{ASCII}
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801 blank \p{PosixBlank} \p{XPosixBlank} \h [1]
802 or \p{HorizSpace} [1]
803 cntrl \p{PosixCntrl} \p{XPosixCntrl} [2]
804 digit \p{PosixDigit} \p{XPosixDigit} \d
805 graph \p{PosixGraph} \p{XPosixGraph} [3]
806 lower \p{PosixLower} \p{XPosixLower}
807 print \p{PosixPrint} \p{XPosixPrint} [4]
808 punct \p{PosixPunct} \p{XPosixPunct} [5]
809 \p{PerlSpace} \p{XPerlSpace} \s [6]
810 space \p{PosixSpace} \p{XPosixSpace} [6]
811 upper \p{PosixUpper} \p{XPosixUpper}
812 word \p{PosixWord} \p{XPosixWord} \w
82206b5e 813 xdigit \p{PosixXDigit} \p{XPosixXDigit}
8a118206
RGS
814
815=over 4
816
ea449505
KW
817=item [1]
818
819C<\p{Blank}> and C<\p{HorizSpace}> are synonyms.
820
821=item [2]
8a118206 822
ea449505 823Control characters don't produce output as such, but instead usually control
b6538e4f 824the terminal somehow: for example, newline and backspace are control characters.
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825On ASCII platforms, in the ASCII range, characters whose code points are
826between 0 and 31 inclusive, plus 127 (C<DEL>) are control characters; on
827EBCDIC platforms, their counterparts are control characters.
8a118206 828
ea449505 829=item [3]
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830
831Any character that is I<graphical>, that is, visible. This class consists
b6538e4f 832of all alphanumeric characters and all punctuation characters.
8a118206 833
ea449505 834=item [4]
8a118206 835
b6538e4f
TC
836All printable characters, which is the set of all graphical characters
837plus those whitespace characters which are not also controls.
ea449505 838
b6dac59a 839=item [5]
ea449505 840
b6538e4f 841C<\p{PosixPunct}> and C<[[:punct:]]> in the ASCII range match all
ea449505
KW
842non-controls, non-alphanumeric, non-space characters:
843C<[-!"#$%&'()*+,./:;<=E<gt>?@[\\\]^_`{|}~]> (although if a locale is in effect,
844it could alter the behavior of C<[[:punct:]]>).
845
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846The similarly named property, C<\p{Punct}>, matches a somewhat different
847set in the ASCII range, namely
0be9b861
KW
848C<[-!"#%&'()*,./:;?@[\\\]_{}]>. That is, it is missing the nine
849characters C<[$+E<lt>=E<gt>^`|~]>.
6c5a041f
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850This is because Unicode splits what POSIX considers to be punctuation into two
851categories, Punctuation and Symbols.
852
e2cfb18c 853C<\p{XPosixPunct}> and (under Unicode rules) C<[[:punct:]]>, match what
765fa144
KW
854C<\p{PosixPunct}> matches in the ASCII range, plus what C<\p{Punct}>
855matches. This is different than strictly matching according to
856C<\p{Punct}>. Another way to say it is that
82206b5e
KW
857if Unicode rules are in effect, C<[[:punct:]]> matches all characters
858that Unicode considers punctuation, plus all ASCII-range characters that
859Unicode considers symbols.
8a118206 860
ea449505 861=item [6]
8a118206 862
7fa2fdc0 863C<\p{XPerlSpace}> and C<\p{Space}> match identically starting with Perl
d28d8023 864v5.18. In earlier versions, these differ only in that in non-locale
7fa2fdc0 865matching, C<\p{XPerlSpace}> does not match the vertical tab, C<\cK>.
d28d8023 866Same for the two ASCII-only range forms.
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867
868=back
869
ab6199be
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870There are various other synonyms that can be used besides the names
871listed in the table. For example, C<\p{PosixAlpha}> can be written as
872C<\p{Alpha}>. All are listed in
d66e1f56 873L<perluniprops/Properties accessible through \p{} and \P{}>.
ab6199be
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874
875Both the C<\p> counterparts always assume Unicode rules are in effect.
876On ASCII platforms, this means they assume that the code points from 128
877to 255 are Latin-1, and that means that using them under locale rules is
878unwise unless the locale is guaranteed to be Latin-1 or UTF-8. In contrast, the
879POSIX character classes are useful under locale rules. They are
880affected by the actual rules in effect, as follows:
881
882=over
883
884=item If the C</a> modifier, is in effect ...
885
886Each of the POSIX classes matches exactly the same as their ASCII-range
887counterparts.
888
889=item otherwise ...
890
891=over
892
893=item For code points above 255 ...
894
895The POSIX class matches the same as its Full-range counterpart.
896
897=item For code points below 256 ...
898
899=over
900
901=item if locale rules are in effect ...
902
a145a423
KW
903The POSIX class matches according to the locale, except:
904
905=over
906
907=item C<word>
908
909also includes the platform's native underscore character, no matter what
8129baca 910the locale is.
ab6199be 911
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912=item C<ascii>
913
914on platforms that don't have the POSIX C<ascii> extension, this matches
915just the platform's native ASCII-range characters.
916
917=item C<blank>
918
919on platforms that don't have the POSIX C<blank> extension, this matches
920just the platform's native tab and space characters.
921
922=back
923
4b9734bf 924=item if Unicode rules are in effect ...
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925
926The POSIX class matches the same as the Full-range counterpart.
927
928=item otherwise ...
929
930The POSIX class matches the same as the ASCII range counterpart.
931
932=back
933
934=back
935
936=back
937
938Which rules apply are determined as described in
939L<perlre/Which character set modifier is in effect?>.
940
941It is proposed to change this behavior in a future release of Perl so that
942whether or not Unicode rules are in effect would not change the
4b9734bf 943behavior: Outside of locale, the POSIX classes
ab6199be
KW
944would behave like their ASCII-range counterparts. If you wish to
945comment on this proposal, send email to C<perl5-porters@perl.org>.
cbc24f92 946
1f59b283 947=head4 Negation of POSIX character classes
ea449505 948X<character class, negation>
8a118206
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949
950A Perl extension to the POSIX character class is the ability to
951negate it. This is done by prefixing the class name with a caret (C<^>).
952Some examples:
953
ea449505
KW
954 POSIX ASCII-range Full-range backslash
955 Unicode Unicode sequence
956 -----------------------------------------------------
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KW
957 [[:^digit:]] \P{PosixDigit} \P{XPosixDigit} \D
958 [[:^space:]] \P{PosixSpace} \P{XPosixSpace}
959 \P{PerlSpace} \P{XPerlSpace} \S
960 [[:^word:]] \P{PerlWord} \P{XPosixWord} \W
961
765fa144 962The backslash sequence can mean either ASCII- or Full-range Unicode,
82206b5e 963depending on various factors as described in L<perlre/Which character set modifier is in effect?>.
8a118206
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964
965=head4 [= =] and [. .]
966
b6538e4f 967Perl recognizes the POSIX character classes C<[=class=]> and
82206b5e 968C<[.class.]>, but does not (yet?) support them. Any attempt to use
b6538e4f 969either construct raises an exception.
8a118206
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970
971=head4 Examples
972
973 /[[:digit:]]/ # Matches a character that is a digit.
974 /[01[:lower:]]/ # Matches a character that is either a
975 # lowercase letter, or '0' or '1'.
c1c4ae3a 976 /[[:digit:][:^xdigit:]]/ # Matches a character that can be anything
bc943be5
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977 # except the letters 'a' to 'f' and 'A' to
978 # 'F'. This is because the main character
979 # class is composed of two POSIX character
980 # classes that are ORed together, one that
981 # matches any digit, and the other that
982 # matches anything that isn't a hex digit.
983 # The OR adds the digits, leaving only the
984 # letters 'a' to 'f' and 'A' to 'F' excluded.
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985
986=head3 Extended Bracketed Character Classes
987X<character class>
988X<set operations>
989
990This is a fancy bracketed character class that can be used for more
991readable and less error-prone classes, and to perform set operations,
992such as intersection. An example is
993
994 /(?[ \p{Thai} & \p{Digit} ])/
995
996This will match all the digit characters that are in the Thai script.
997
998This is an experimental feature available starting in 5.18, and is
999subject to change as we gain field experience with it. Any attempt to
1000use it will raise a warning, unless disabled via
1001
1002 no warnings "experimental::regex_sets";
1003
1004Comments on this feature are welcome; send email to
1005C<perl5-porters@perl.org>.
1006
1007We can extend the example above:
1008
1009 /(?[ ( \p{Thai} + \p{Lao} ) & \p{Digit} ])/
1010
1011This matches digits that are in either the Thai or Laotian scripts.
1012
1013Notice the white space in these examples. This construct always has
d66e1f56 1014the C<E<sol>x> modifier turned on within it.
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1015
1016The available binary operators are:
1017
1018 & intersection
1019 + union
1020 | another name for '+', hence means union
1021 - subtraction (the result matches the set consisting of those
1022 code points matched by the first operand, excluding any that
1023 are also matched by the second operand)
1024 ^ symmetric difference (the union minus the intersection). This
1025 is like an exclusive or, in that the result is the set of code
1026 points that are matched by either, but not both, of the
1027 operands.
1028
1029There is one unary operator:
1030
1031 ! complement
1032
1033All the binary operators left associate, and are of equal precedence.
1034The unary operator right associates, and has higher precedence. Use
1035parentheses to override the default associations. Some feedback we've
1036received indicates a desire for intersection to have higher precedence
1037than union. This is something that feedback from the field may cause us
1038to change in future releases; you may want to parenthesize copiously to
1039avoid such changes affecting your code, until this feature is no longer
1040considered experimental.
1041
1042The main restriction is that everything is a metacharacter. Thus,
1043you cannot refer to single characters by doing something like this:
1044
1045 /(?[ a + b ])/ # Syntax error!
1046
1047The easiest way to specify an individual typable character is to enclose
1048it in brackets:
1049
1050 /(?[ [a] + [b] ])/
1051
1052(This is the same thing as C<[ab]>.) You could also have said the
1053equivalent:
1054
1055 /(?[[ a b ]])/
1056
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1057(You can, of course, specify single characters by using, C<\x{...}>,
1058C<\N{...}>, etc.)
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1059
1060This last example shows the use of this construct to specify an ordinary
1061bracketed character class without additional set operations. Note the
1062white space within it; C<E<sol>x> is turned on even within bracketed
1063character classes, except you can't have comments inside them. Hence,
1064
1065 (?[ [#] ])
1066
1067matches the literal character "#". To specify a literal white space character,
1068you can escape it with a backslash, like:
1069
1070 /(?[ [ a e i o u \ ] ])/
1071
1072This matches the English vowels plus the SPACE character.
1073All the other escapes accepted by normal bracketed character classes are
1074accepted here as well; but unrecognized escapes that generate warnings
1075in normal classes are fatal errors here.
1076
1077All warnings from these class elements are fatal, as well as some
1078practices that don't currently warn. For example you cannot say
1079
1080 /(?[ [ \xF ] ])/ # Syntax error!
1081
1082You have to have two hex digits after a braceless C<\x> (use a leading
1083zero to make two). These restrictions are to lower the incidence of
1084typos causing the class to not match what you thought it would.
1085
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1086If a regular bracketed character class contains a C<\p{}> or C<\P{}> and
1087is matched against a non-Unicode code point, a warning may be
1088raised, as the result is not Unicode-defined. No such warning will come
1089when using this extended form.
1090
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1091The final difference between regular bracketed character classes and
1092these, is that it is not possible to get these to match a
1093multi-character fold. Thus,
1094
1095 /(?[ [\xDF] ])/iu
1096
1097does not match the string C<ss>.
1098
1099You don't have to enclose POSIX class names inside double brackets,
1100hence both of the following work:
1101
1102 /(?[ [:word:] - [:lower:] ])/
1103 /(?[ [[:word:]] - [[:lower:]] ])/
1104
1105Any contained POSIX character classes, including things like C<\w> and C<\D>
1106respect the C<E<sol>a> (and C<E<sol>aa>) modifiers.
1107
1108C<< (?[ ]) >> is a regex-compile-time construct. Any attempt to use
1109something which isn't knowable at the time the containing regular
1110expression is compiled is a fatal error. In practice, this means
11a9b3e0 1111just three limitations:
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1112
1113=over 4
1114
1115=item 1
1116
1117This construct cannot be used within the scope of
1118C<use locale> (or the C<E<sol>l> regex modifier).
1119
1120=item 2
1121
1122Any
1123L<user-defined property|perlunicode/"User-Defined Character Properties">
1124used must be already defined by the time the regular expression is
1125compiled (but note that this construct can be used instead of such
1126properties).
1127
1128=item 3
1129
1130A regular expression that otherwise would compile
1131using C<E<sol>d> rules, and which uses this construct will instead
1132use C<E<sol>u>. Thus this construct tells Perl that you don't want
1133C<E<sol>d> rules for the entire regular expression containing it.
1134
1135=back
1136
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1137Note that skipping white space applies only to the interior of this
1138construct. There must not be any space between any of the characters
1139that form the initial C<(?[>. Nor may there be space between the
1140closing C<])> characters.
1141
11a9b3e0 1142Just as in all regular expressions, the pattern can be built up by
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1143including variables that are interpolated at regex compilation time.
1144Care must be taken to ensure that you are getting what you expect. For
1145example:
1146
1147 my $thai_or_lao = '\p{Thai} + \p{Lao}';
1148 ...
1149 qr/(?[ \p{Digit} & $thai_or_lao ])/;
1150
1151compiles to
1152
1153 qr/(?[ \p{Digit} & \p{Thai} + \p{Lao} ])/;
1154
1155But this does not have the effect that someone reading the code would
1156likely expect, as the intersection applies just to C<\p{Thai}>,
1157excluding the Laotian. Pitfalls like this can be avoided by
1158parenthesizing the component pieces:
1159
1160 my $thai_or_lao = '( \p{Thai} + \p{Lao} )';
1161
1162But any modifiers will still apply to all the components:
1163
1164 my $lower = '\p{Lower} + \p{Digit}';
1165 qr/(?[ \p{Greek} & $lower ])/i;
1166
1167matches upper case things. You can avoid surprises by making the
1168components into instances of this construct by compiling them:
1169
1170 my $thai_or_lao = qr/(?[ \p{Thai} + \p{Lao} ])/;
1171 my $lower = qr/(?[ \p{Lower} + \p{Digit} ])/;
1172
1173When these are embedded in another pattern, what they match does not
1174change, regardless of parenthesization or what modifiers are in effect
1175in that outer pattern.
1176
1177Due to the way that Perl parses things, your parentheses and brackets
1178may need to be balanced, even including comments. If you run into any
1179examples, please send them to C<perlbug@perl.org>, so that we can have a
1180concrete example for this man page.
1181
1182We may change it so that things that remain legal uses in normal bracketed
1183character classes might become illegal within this experimental
1184construct. One proposal, for example, is to forbid adjacent uses of the
1185same character, as in C<(?[ [aa] ])>. The motivation for such a change
1186is that this usage is likely a typo, as the second "a" adds nothing.