1=head1 NAME 2X<character class> 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 12classes in Perl regular expressions. 13 14A character class is a way of denoting a set of characters 15in such a way that one character of the set is matched. 16It's important to remember that: matching a character class 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 21expressions: the dot, backslash sequences, and the form enclosed in square 22brackets. Keep in mind, though, that often the term "character class" is used 23to mean just the bracketed form. Certainly, most Perl documentation does that. 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 29character, except for the newline. That default can be changed to 30add matching the newline by using the I<single line> modifier: 31for the entire regular expression with the C</s> modifier, or 32locally with C<(?s)> (and even globally within the scope of 33L<C<use re '/s'>|re/'E<sol>flags' mode>). (The C<L</\N>> backslash 34sequence, described 35below, matches any character except newline without regard to the 36I<single line> modifier.) 37 38Here are some examples: 39 40 "a" =~ /./ # Match 41 "." =~ /./ # Match 42 "" =~ /./ # No match (dot has to match a character) 43 "\n" =~ /./ # No match (dot does not match a newline) 44 "\n" =~ /./s # Match (global 'single line' modifier) 45 "\n" =~ /(?s:.)/ # Match (local 'single line' modifier) 46 "ab" =~ /^.$/ # No match (dot matches one character) 47 48=head2 Backslash sequences 49X<\w> X<\W> X<\s> X<\S> X<\d> X<\D> X<\p> X<\P> 50X<\N> X<\v> X<\V> X<\h> X<\H> 51X<word> X<whitespace> 52 53A backslash sequence is a sequence of characters, the first one of which is a 54backslash. Perl ascribes special meaning to many such sequences, and some of 55these are character classes. That is, they match a single character each, 56provided that the character belongs to the specific set of characters defined 57by the sequence. 58 59Here's a list of the backslash sequences that are character classes. They 60are discussed in more detail below. (For the backslash sequences that aren't 61character classes, see L<perlrebackslash>.) 62 63 \d Match a decimal digit character. 64 \D Match a non-decimal-digit character. 65 \w Match a "word" character. 66 \W Match a non-"word" character. 67 \s Match a whitespace character. 68 \S Match a non-whitespace character. 69 \h Match a horizontal whitespace character. 70 \H Match a character that isn't horizontal whitespace. 71 \v Match a vertical whitespace character. 72 \V Match a character that isn't vertical whitespace. 73 \N Match a character that isn't a newline. 74 \pP, \p{Prop} Match a character that has the given Unicode property. 75 \PP, \P{Prop} Match a character that doesn't have the Unicode property 76 77=head3 \N 78 79C<\N>, available starting in v5.12, like the dot, matches any 80character that is not a newline. The difference is that C<\N> is not influenced 81by the I<single line> regular expression modifier (see L</The dot> above). Note 82that the form C<\N{...}> may mean something completely different. When the 83C<{...}> is a L<quantifier|perlre/Quantifiers>, it means to match a non-newline 84character that many times. For example, C<\N{3}> means to match 3 85non-newlines; C<\N{5,}> means to match 5 or more non-newlines. But if C<{...}> 86is not a legal quantifier, it is presumed to be a named character. See 87L<charnames> for those. For example, none of C<\N{COLON}>, C<\N{4F}>, and 88C<\N{F4}> contain legal quantifiers, so Perl will try to find characters whose 89names are respectively C<COLON>, C<4F>, and C<F4>. 90 91=head3 Digits 92 93C<\d> matches a single character considered to be a decimal I<digit>. 94If the C</a> regular expression modifier is in effect, it matches [0-9]. 95Otherwise, it 96matches anything that is matched by C<\p{Digit}>, which includes [0-9]. 97(An unlikely possible exception is that under locale matching rules, the 98current locale might not have C<[0-9]> matched by C<\d>, and/or might match 99other characters whose code point is less than 256. The only such locale 100definitions that are legal would be to match C<[0-9]> plus another set of 10110 consecutive digit characters; anything else would be in violation of 102the C language standard, but Perl doesn't currently assume anything in 103regard to this.) 104 105What this means is that unless the C</a> modifier is in effect C<\d> not 106only matches the digits '0' - '9', but also Arabic, Devanagari, and 107digits from other languages. This may cause some confusion, and some 108security issues. 109 110Some digits that C<\d> matches look like some of the [0-9] ones, but 111have different values. For example, BENGALI DIGIT FOUR (U+09EA) looks 112very much like an ASCII DIGIT EIGHT (U+0038), and LEPCHA DIGIT SIX 113(U+1C46) looks very much like an ASCII DIGIT FIVE (U+0035). An 114application that 115is expecting only the ASCII digits might be misled, or if the match is 116C<\d+>, the matched string might contain a mixture of digits from 117different writing systems that look like they signify a number different 118than they actually do. L<Unicode::UCD/num()> can 119be used to safely 120calculate the value, returning C<undef> if the input string contains 121such a mixture. Otherwise, for example, a displayed price might be 122deliberately different than it appears. 123 124What C<\p{Digit}> means (and hence C<\d> except under the C</a> 125modifier) is C<\p{General_Category=Decimal_Number}>, or synonymously, 126C<\p{General_Category=Digit}>. Starting with Unicode version 4.1, this 127is the same set of characters matched by C<\p{Numeric_Type=Decimal}>. 128But Unicode also has a different property with a similar name, 129C<\p{Numeric_Type=Digit}>, which matches a completely different set of 130characters. These characters are things such as C<CIRCLED DIGIT ONE> 131or subscripts, or are from writing systems that lack all ten digits. 132 133The design intent is for C<\d> to exactly match the set of characters 134that can safely be used with "normal" big-endian positional decimal 135syntax, where, for example 123 means one 'hundred', plus two 'tens', 136plus three 'ones'. This positional notation does not necessarily apply 137to characters that match the other type of "digit", 138C<\p{Numeric_Type=Digit}>, and so C<\d> doesn't match them. 139 140The Tamil digits (U+0BE6 - U+0BEF) can also legally be 141used in old-style Tamil numbers in which they would appear no more than 142one in a row, separated by characters that mean "times 10", "times 100", 143etc. (See L<http://www.unicode.org/notes/tn21>.) 144 145Any character not matched by C<\d> is matched by C<\D>. 146 147=head3 Word characters 148 149A C<\w> matches a single alphanumeric character (an alphabetic character, or a 150decimal digit); or a connecting punctuation character, such as an 151underscore ("_"); or a "mark" character (like some sort of accent) that 152attaches to one of those. It does not match a whole word. To match a 153whole word, use C<\w+>. This isn't the same thing as matching an 154English word, but in the ASCII range it is the same as a string of 155Perl-identifier characters. 156 157=over 158 159=item If the C</a> modifier is in effect ... 160 161C<\w> matches the 63 characters [a-zA-Z0-9_]. 162 163=item otherwise ... 164 165=over 166 167=item For code points above 255 ... 168 169C<\w> matches the same as C<\p{Word}> matches in this range. That is, 170it matches Thai letters, Greek letters, etc. This includes connector 171punctuation (like the underscore) which connect two words together, or 172diacritics, such as a C<COMBINING TILDE> and the modifier letters, which 173are generally used to add auxiliary markings to letters. 174 175=item For code points below 256 ... 176 177=over 178 179=item if locale rules are in effect ... 180 181C<\w> matches the platform's native underscore character plus whatever 182the locale considers to be alphanumeric. 183 184=item if, instead, Unicode rules are in effect ... 185 186C<\w> matches exactly what C<\p{Word}> matches. 187 188=item otherwise ... 189 190C<\w> matches [a-zA-Z0-9_]. 191 192=back 193 194=back 195 196=back 197 198Which rules apply are determined as described in L<perlre/Which character set modifier is in effect?>. 199 200There are a number of security issues with the full Unicode list of word 201characters. See L<http://unicode.org/reports/tr36>. 202 203Also, for a somewhat finer-grained set of characters that are in programming 204language identifiers beyond the ASCII range, you may wish to instead use the 205more customized L</Unicode Properties>, C<\p{ID_Start}>, 206C<\p{ID_Continue}>, C<\p{XID_Start}>, and C<\p{XID_Continue}>. See 207L<http://unicode.org/reports/tr31>. 208 209Any character not matched by C<\w> is matched by C<\W>. 210 211=head3 Whitespace 212 213C<\s> matches any single character considered whitespace. 214 215=over 216 217=item If the C</a> modifier is in effect ... 218 219In all Perl versions, C<\s> matches the 5 characters [\t\n\f\r ]; that 220is, the horizontal tab, 221the newline, the form feed, the carriage return, and the space. 222Starting in Perl v5.18, it also matches the vertical tab, C<\cK>. 223See note C<[1]> below for a discussion of this. 224 225=item otherwise ... 226 227=over 228 229=item For code points above 255 ... 230 231C<\s> matches exactly the code points above 255 shown with an "s" column 232in the table below. 233 234=item For code points below 256 ... 235 236=over 237 238=item if locale rules are in effect ... 239 240C<\s> matches whatever the locale considers to be whitespace. 241 242=item if, instead, Unicode rules are in effect ... 243 244C<\s> matches exactly the characters shown with an "s" column in the 245table below. 246 247=item otherwise ... 248 249C<\s> matches [\t\n\f\r ] and, starting in Perl 250v5.18, the vertical tab, C<\cK>. 251(See note C<[1]> below for a discussion of this.) 252Note that this list doesn't include the non-breaking space. 253 254=back 255 256=back 257 258=back 259 260Which rules apply are determined as described in L<perlre/Which character set modifier is in effect?>. 261 262Any character not matched by C<\s> is matched by C<\S>. 263 264C<\h> matches any character considered horizontal whitespace; 265this includes the platform's space and tab characters and several others 266listed in the table below. C<\H> matches any character 267not considered horizontal whitespace. They use the platform's native 268character set, and do not consider any locale that may otherwise be in 269use. 270 271C<\v> matches any character considered vertical whitespace; 272this includes the platform's carriage return and line feed characters (newline) 273plus several other characters, all listed in the table below. 274C<\V> matches any character not considered vertical whitespace. 275They use the platform's native character set, and do not consider any 276locale that may otherwise be in use. 277 278C<\R> matches anything that can be considered a newline under Unicode 279rules. It can match a multi-character sequence. It cannot be used inside 280a bracketed character class; use C<\v> instead (vertical whitespace). 281It uses the platform's 282native character set, and does not consider any locale that may 283otherwise be in use. 284Details are discussed in L<perlrebackslash>. 285 286Note that unlike C<\s> (and C<\d> and C<\w>), C<\h> and C<\v> always match 287the same characters, without regard to other factors, such as the active 288locale or whether the source string is in UTF-8 format. 289 290One might think that C<\s> is equivalent to C<[\h\v]>. This is indeed true 291starting in Perl v5.18, but prior to that, the sole difference was that the 292vertical tab (C<"\cK">) was not matched by C<\s>. 293 294The following table is a complete listing of characters matched by 295C<\s>, C<\h> and C<\v> as of Unicode 6.3. 296 297The first column gives the Unicode code point of the character (in hex format), 298the second column gives the (Unicode) name. The third column indicates 299by which class(es) the character is matched (assuming no locale is in 300effect that changes the C<\s> matching). 301 302 0x0009 CHARACTER TABULATION h s 303 0x000a LINE FEED (LF) vs 304 0x000b LINE TABULATION vs [1] 305 0x000c FORM FEED (FF) vs 306 0x000d CARRIAGE RETURN (CR) vs 307 0x0020 SPACE h s 308 0x0085 NEXT LINE (NEL) vs [2] 309 0x00a0 NO-BREAK SPACE h s [2] 310 0x1680 OGHAM SPACE MARK h s 311 0x2000 EN QUAD h s 312 0x2001 EM QUAD h s 313 0x2002 EN SPACE h s 314 0x2003 EM SPACE h s 315 0x2004 THREE-PER-EM SPACE h s 316 0x2005 FOUR-PER-EM SPACE h s 317 0x2006 SIX-PER-EM SPACE h s 318 0x2007 FIGURE SPACE h s 319 0x2008 PUNCTUATION SPACE h s 320 0x2009 THIN SPACE h s 321 0x200a HAIR SPACE h s 322 0x2028 LINE SEPARATOR vs 323 0x2029 PARAGRAPH SEPARATOR vs 324 0x202f NARROW NO-BREAK SPACE h s 325 0x205f MEDIUM MATHEMATICAL SPACE h s 326 0x3000 IDEOGRAPHIC SPACE h s 327 328=over 4 329 330=item [1] 331 332Prior to Perl v5.18, C<\s> did not match the vertical tab. 333C<[^\S\cK]> (obscurely) matches what C<\s> traditionally did. 334 335=item [2] 336 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>. 340 341=back 342 343=head3 Unicode Properties 344 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}>, 350which means to match if the property "name" for the character has that particular 351"value". 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 355has the short form I<Ll>. They need the braces, so are written as C</\p{Ll}/> or 356C</\p{Lowercase_Letter}/>, or C</\p{General_Category=Lowercase_Letter}/> 357(the underscores are optional). 358C</\pLl/> is valid, but means something different. 359It matches a two character string: a letter (Unicode property C<\pL>), 360followed by a lowercase C<l>. 361 362What a Unicode property matches is never subject to locale rules, and 363if locale rules are not otherwise in effect, the use of a Unicode 364property will force the regular expression into using Unicode rules, if 365it isn't already. 366 367Note that almost all properties are immune to case-insensitive matching. 368That is, adding a C</i> regular expression modifier does not change what 369they match. There are two sets that are affected. The first set is 370C<Uppercase_Letter>, 371C<Lowercase_Letter>, 372and C<Titlecase_Letter>, 373all of which match C<Cased_Letter> under C</i> matching. 374The second set is 375C<Uppercase>, 376C<Lowercase>, 377and C<Titlecase>, 378all of which match C<Cased> under C</i> matching. 379(The difference between these sets is that some things, such as Roman 380numerals, come in both upper and lower case, so they are C<Cased>, but 381aren't considered to be letters, so they aren't C<Cased_Letter>s. They're 382actually C<Letter_Number>s.) 383This set also includes its subsets C<PosixUpper> and C<PosixLower>, both 384of which under C</i> match C<PosixAlpha>. 385 386For more details on Unicode properties, see L<perlunicode/Unicode 387Character Properties>; for a 388complete list of possible properties, see 389L<perluniprops/Properties accessible through \p{} and \P{}>, 390which notes all forms that have C</i> differences. 391It is also possible to define your own properties. This is discussed in 392L<perlunicode/User-Defined Character Properties>. 393 394Unicode properties are defined (surprise!) only on Unicode code points. 395Starting in v5.20, when matching against C<\p> and C<\P>, Perl treats 396non-Unicode code points (those above the legal Unicode maximum of 3970x10FFFF) as if they were typical unassigned Unicode code points. 398 399Prior to v5.20, Perl raised a warning and made all matches fail on 400non-Unicode code points. This could be somewhat surprising: 401 402 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails on Perls < v5.20. 403 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Also fails on Perls 404 # < v5.20 405 406Even though these two matches might be thought of as complements, until 407v5.20 they were so only on Unicode code points. 408 409Starting in perl v5.30, wildcards are allowed in Unicode property 410values. See L<perlunicode/Wildcards in Property Values>. 411 412=head4 Examples 413 414 "a" =~ /\w/ # Match, "a" is a 'word' character. 415 "7" =~ /\w/ # Match, "7" is a 'word' character as well. 416 "a" =~ /\d/ # No match, "a" isn't a digit. 417 "7" =~ /\d/ # Match, "7" is a digit. 418 " " =~ /\s/ # Match, a space is whitespace. 419 "a" =~ /\D/ # Match, "a" is a non-digit. 420 "7" =~ /\D/ # No match, "7" is not a non-digit. 421 " " =~ /\S/ # No match, a space is not non-whitespace. 422 423 " " =~ /\h/ # Match, space is horizontal whitespace. 424 " " =~ /\v/ # No match, space is not vertical whitespace. 425 "\r" =~ /\v/ # Match, a return is vertical whitespace. 426 427 "a" =~ /\pL/ # Match, "a" is a letter. 428 "a" =~ /\p{Lu}/ # No match, /\p{Lu}/ matches upper case letters. 429 430 "\x{0e0b}" =~ /\p{Thai}/ # Match, \x{0e0b} is the character 431 # 'THAI CHARACTER SO SO', and that's in 432 # Thai Unicode class. 433 "a" =~ /\P{Lao}/ # Match, as "a" is not a Laotian character. 434 435It is worth emphasizing that C<\d>, C<\w>, etc, match single characters, not 436complete numbers or words. To match a number (that consists of digits), 437use C<\d+>; to match a word, use C<\w+>. But be aware of the security 438considerations in doing so, as mentioned above. 439 440=head2 Bracketed Character Classes 441 442The third form of character class you can use in Perl regular expressions 443is the bracketed character class. In its simplest form, it lists the characters 444that may be matched, surrounded by square brackets, like this: C<[aeiou]>. 445This matches one of C<a>, C<e>, C<i>, C<o> or C<u>. Like the other 446character classes, exactly one character is matched.* To match 447a longer string consisting of characters mentioned in the character 448class, follow the character class with a L<quantifier|perlre/Quantifiers>. For 449instance, C<[aeiou]+> matches one or more lowercase English vowels. 450 451Repeating a character in a character class has no 452effect; it's considered to be in the set only once. 453 454Examples: 455 456 "e" =~ /[aeiou]/ # Match, as "e" is listed in the class. 457 "p" =~ /[aeiou]/ # No match, "p" is not listed in the class. 458 "ae" =~ /^[aeiou]$/ # No match, a character class only matches 459 # a single character. 460 "ae" =~ /^[aeiou]+$/ # Match, due to the quantifier. 461 462 ------- 463 464* There are two exceptions to a bracketed character class matching a 465single character only. Each requires special handling by Perl to make 466things work: 467 468=over 469 470=item * 471 472When the class is to match caselessly under C</i> matching rules, and a 473character that is explicitly mentioned inside the class matches a 474multiple-character sequence caselessly under Unicode rules, the class 475will also match that sequence. For example, Unicode says that the 476letter C<LATIN SMALL LETTER SHARP S> should match the sequence C<ss> 477under C</i> rules. Thus, 478 479 'ss' =~ /\A\N{LATIN SMALL LETTER SHARP S}\z/i # Matches 480 'ss' =~ /\A[aeioust\N{LATIN SMALL LETTER SHARP S}]\z/i # Matches 481 482For this to happen, the class must not be inverted (see L</Negation>) 483and the character must be explicitly specified, and not be part of a 484multi-character range (not even as one of its endpoints). (L</Character 485Ranges> will be explained shortly.) Therefore, 486 487 'ss' =~ /\A[\0-\x{ff}]\z/ui # Doesn't match 488 'ss' =~ /\A[\0-\N{LATIN SMALL LETTER SHARP S}]\z/ui # No match 489 'ss' =~ /\A[\xDF-\xDF]\z/ui # Matches on ASCII platforms, since 490 # \xDF is LATIN SMALL LETTER SHARP S, 491 # and the range is just a single 492 # element 493 494Note that it isn't a good idea to specify these types of ranges anyway. 495 496=item * 497 498Some names known to C<\N{...}> refer to a sequence of multiple characters, 499instead of the usual single character. When one of these is included in 500the class, the entire sequence is matched. For example, 501 502 "\N{TAMIL LETTER KA}\N{TAMIL VOWEL SIGN AU}" 503 =~ / ^ [\N{TAMIL SYLLABLE KAU}] $ /x; 504 505matches, because C<\N{TAMIL SYLLABLE KAU}> is a named sequence 506consisting of the two characters matched against. Like the other 507instance where a bracketed class can match multiple characters, and for 508similar reasons, the class must not be inverted, and the named sequence 509may not appear in a range, even one where it is both endpoints. If 510these happen, it is a fatal error if the character class is within the 511scope of L<C<use re 'strict>|re/'strict' mode>, or within an extended 512L<C<(?[...])>|/Extended Bracketed Character Classes> class; otherwise 513only the first code point is used (with a C<regexp>-type warning 514raised). 515 516=back 517 518=head3 Special Characters Inside a Bracketed Character Class 519 520Most characters that are meta characters in regular expressions (that 521is, characters that carry a special meaning like C<.>, C<*>, or C<(>) lose 522their special meaning and can be used inside a character class without 523the need to escape them. For instance, C<[()]> matches either an opening 524parenthesis, or a closing parenthesis, and the parens inside the character 525class don't group or capture. Be aware that, unless the pattern is 526evaluated in single-quotish context, variable interpolation will take 527place before the bracketed class is parsed: 528 529 $, = "\t| "; 530 $a =~ m'[$,]'; # single-quotish: matches '$' or ',' 531 $a =~ q{[$,]}' # same 532 $a =~ m/[$,]/; # double-quotish: matches "\t", "|", or " " 533 534Characters that may carry a special meaning inside a character class are: 535C<\>, C<^>, C<->, C<[> and C<]>, and are discussed below. They can be 536escaped with a backslash, although this is sometimes not needed, in which 537case the backslash may be omitted. 538 539The sequence C<\b> is special inside a bracketed character class. While 540outside the character class, C<\b> is an assertion indicating a point 541that does not have either two word characters or two non-word characters 542on either side, inside a bracketed character class, C<\b> matches a 543backspace character. 544 545The sequences 546C<\a>, 547C<\c>, 548C<\e>, 549C<\f>, 550C<\n>, 551C<\N{I<NAME>}>, 552C<\N{U+I<hex char>}>, 553C<\r>, 554C<\t>, 555and 556C<\x> 557are also special and have the same meanings as they do outside a 558bracketed character class. 559 560Also, a backslash followed by two or three octal digits is considered an octal 561number. 562 563A C<[> is not special inside a character class, unless it's the start of a 564POSIX character class (see L</POSIX Character Classes> below). It normally does 565not need escaping. 566 567A C<]> is normally either the end of a POSIX character class (see 568L</POSIX Character Classes> below), or it signals the end of the bracketed 569character class. If you want to include a C<]> in the set of characters, you 570must generally escape it. 571 572However, if the C<]> is the I<first> (or the second if the first 573character is a caret) character of a bracketed character class, it 574does not denote the end of the class (as you cannot have an empty class) 575and is considered part of the set of characters that can be matched without 576escaping. 577 578Examples: 579 580 "+" =~ /[+?*]/ # Match, "+" in a character class is not special. 581 "\cH" =~ /[\b]/ # Match, \b inside in a character class 582 # is equivalent to a backspace. 583 "]" =~ /[][]/ # Match, as the character class contains 584 # both [ and ]. 585 "[]" =~ /[[]]/ # Match, the pattern contains a character class 586 # containing just [, and the character class is 587 # followed by a ]. 588 589=head3 Bracketed Character Classes and the C</xx> pattern modifier 590 591Normally SPACE and TAB characters have no special meaning inside a 592bracketed character class; they are just added to the list of characters 593matched by the class. But if the L<C</xx>|perlre/E<sol>x and E<sol>xx> 594pattern modifier is in effect, they are generally ignored and can be 595added to improve readability. They can't be added in the middle of a 596single construct: 597 598 / [ \x{10 FFFF} ] /xx # WRONG! 599 600The SPACE in the middle of the hex constant is illegal. 601 602To specify a literal SPACE character, you can escape it with a 603backslash, like: 604 605 /[ a e i o u \ ]/xx 606 607This matches the English vowels plus the SPACE character. 608 609For clarity, you should already have been using C<\t> to specify a 610literal tab, and C<\t> is unaffected by C</xx>. 611 612=head3 Character Ranges 613 614It is not uncommon to want to match a range of characters. Luckily, instead 615of listing all characters in the range, one may use the hyphen (C<->). 616If inside a bracketed character class you have two characters separated 617by a hyphen, it's treated as if all characters between the two were in 618the class. For instance, C<[0-9]> matches any ASCII digit, and C<[a-m]> 619matches any lowercase letter from the first half of the ASCII alphabet. 620 621Note that the two characters on either side of the hyphen are not 622necessarily both letters or both digits. Any character is possible, 623although not advisable. C<['-?]> contains a range of characters, but 624most people will not know which characters that means. Furthermore, 625such ranges may lead to portability problems if the code has to run on 626a platform that uses a different character set, such as EBCDIC. 627 628If a hyphen in a character class cannot syntactically be part of a range, for 629instance because it is the first or the last character of the character class, 630or if it immediately follows a range, the hyphen isn't special, and so is 631considered a character to be matched literally. If you want a hyphen in 632your set of characters to be matched and its position in the class is such 633that it could be considered part of a range, you must escape that hyphen 634with a backslash. 635 636Examples: 637 638 [a-z] # Matches a character that is a lower case ASCII letter. 639 [a-fz] # Matches any letter between 'a' and 'f' (inclusive) or 640 # the letter 'z'. 641 [-z] # Matches either a hyphen ('-') or the letter 'z'. 642 [a-f-m] # Matches any letter between 'a' and 'f' (inclusive), the 643 # hyphen ('-'), or the letter 'm'. 644 ['-?] # Matches any of the characters '()*+,-./0123456789:;<=>? 645 # (But not on an EBCDIC platform). 646 [\N{APOSTROPHE}-\N{QUESTION MARK}] 647 # Matches any of the characters '()*+,-./0123456789:;<=>? 648 # even on an EBCDIC platform. 649 [\N{U+27}-\N{U+3F}] # Same. (U+27 is "'", and U+3F is "?") 650 651As the final two examples above show, you can achieve portability to 652non-ASCII platforms by using the C<\N{...}> form for the range 653endpoints. These indicate that the specified range is to be interpreted 654using Unicode values, so C<[\N{U+27}-\N{U+3F}]> means to match 655C<\N{U+27}>, C<\N{U+28}>, C<\N{U+29}>, ..., C<\N{U+3D}>, C<\N{U+3E}>, 656and C<\N{U+3F}>, whatever the native code point versions for those are. 657These are called "Unicode" ranges. If either end is of the C<\N{...}> 658form, the range is considered Unicode. A C<regexp> warning is raised 659under C<S<"use re 'strict'">> if the other endpoint is specified 660non-portably: 661 662 [\N{U+00}-\x09] # Warning under re 'strict'; \x09 is non-portable 663 [\N{U+00}-\t] # No warning; 664 665Both of the above match the characters C<\N{U+00}> C<\N{U+01}>, ... 666C<\N{U+08}>, C<\N{U+09}>, but the C<\x09> looks like it could be a 667mistake so the warning is raised (under C<re 'strict'>) for it. 668 669Perl also guarantees that the ranges C<A-Z>, C<a-z>, C<0-9>, and any 670subranges of these match what an English-only speaker would expect them 671to match on any platform. That is, C<[A-Z]> matches the 26 ASCII 672uppercase letters; 673C<[a-z]> matches the 26 lowercase letters; and C<[0-9]> matches the 10 674digits. Subranges, like C<[h-k]>, match correspondingly, in this case 675just the four letters C<"h">, C<"i">, C<"j">, and C<"k">. This is the 676natural behavior on ASCII platforms where the code points (ordinal 677values) for C<"h"> through C<"k"> are consecutive integers (0x68 through 6780x6B). But special handling to achieve this may be needed on platforms 679with a non-ASCII native character set. For example, on EBCDIC 680platforms, the code point for C<"h"> is 0x88, C<"i"> is 0x89, C<"j"> is 6810x91, and C<"k"> is 0x92. Perl specially treats C<[h-k]> to exclude the 682seven code points in the gap: 0x8A through 0x90. This special handling is 683only invoked when the range is a subrange of one of the ASCII uppercase, 684lowercase, and digit ranges, AND each end of the range is expressed 685either as a literal, like C<"A">, or as a named character (C<\N{...}>, 686including the C<\N{U+...> form). 687 688EBCDIC Examples: 689 690 [i-j] # Matches either "i" or "j" 691 [i-\N{LATIN SMALL LETTER J}] # Same 692 [i-\N{U+6A}] # Same 693 [\N{U+69}-\N{U+6A}] # Same 694 [\x{89}-\x{91}] # Matches 0x89 ("i"), 0x8A .. 0x90, 0x91 ("j") 695 [i-\x{91}] # Same 696 [\x{89}-j] # Same 697 [i-J] # Matches, 0x89 ("i") .. 0xC1 ("J"); special 698 # handling doesn't apply because range is mixed 699 # case 700 701=head3 Negation 702 703It is also possible to instead list the characters you do not want to 704match. You can do so by using a caret (C<^>) as the first character in the 705character class. For instance, C<[^a-z]> matches any character that is not a 706lowercase ASCII letter, which therefore includes more than a million 707Unicode code points. The class is said to be "negated" or "inverted". 708 709This syntax make the caret a special character inside a bracketed character 710class, but only if it is the first character of the class. So if you want 711the caret as one of the characters to match, either escape the caret or 712else don't list it first. 713 714In inverted bracketed character classes, Perl ignores the Unicode rules 715that normally say that named sequence, and certain characters should 716match a sequence of multiple characters use under caseless C</i> 717matching. Following those rules could lead to highly confusing 718situations: 719 720 "ss" =~ /^[^\xDF]+$/ui; # Matches! 721 722This should match any sequences of characters that aren't C<\xDF> nor 723what C<\xDF> matches under C</i>. C<"s"> isn't C<\xDF>, but Unicode 724says that C<"ss"> is what C<\xDF> matches under C</i>. So which one 725"wins"? Do you fail the match because the string has C<ss> or accept it 726because it has an C<s> followed by another C<s>? Perl has chosen the 727latter. (See note in L</Bracketed Character Classes> above.) 728 729Examples: 730 731 "e" =~ /[^aeiou]/ # No match, the 'e' is listed. 732 "x" =~ /[^aeiou]/ # Match, as 'x' isn't a lowercase vowel. 733 "^" =~ /[^^]/ # No match, matches anything that isn't a caret. 734 "^" =~ /[x^]/ # Match, caret is not special here. 735 736=head3 Backslash Sequences 737 738You can put any backslash sequence character class (with the exception of 739C<\N> and C<\R>) inside a bracketed character class, and it will act just 740as if you had put all characters matched by the backslash sequence inside the 741character class. For instance, C<[a-f\d]> matches any decimal digit, or any 742of the lowercase letters between 'a' and 'f' inclusive. 743 744C<\N> within a bracketed character class must be of the forms C<\N{I<name>}> 745or C<\N{U+I<hex char>}>, and NOT be the form that matches non-newlines, 746for the same reason that a dot C<.> inside a bracketed character class loses 747its special meaning: it matches nearly anything, which generally isn't what you 748want to happen. 749 750 751Examples: 752 753 /[\p{Thai}\d]/ # Matches a character that is either a Thai 754 # character, or a digit. 755 /[^\p{Arabic}()]/ # Matches a character that is neither an Arabic 756 # character, nor a parenthesis. 757 758Backslash sequence character classes cannot form one of the endpoints 759of a range. Thus, you can't say: 760 761 /[\p{Thai}-\d]/ # Wrong! 762 763=head3 POSIX Character Classes 764X<character class> X<\p> X<\p{}> 765X<alpha> X<alnum> X<ascii> X<blank> X<cntrl> X<digit> X<graph> 766X<lower> X<print> X<punct> X<space> X<upper> X<word> X<xdigit> 767 768POSIX character classes have the form C<[:class:]>, where I<class> is the 769name, and the C<[:> and C<:]> delimiters. POSIX character classes only appear 770I<inside> bracketed character classes, and are a convenient and descriptive 771way of listing a group of characters. 772 773Be careful about the syntax, 774 775 # Correct: 776 $string =~ /[[:alpha:]]/ 777 778 # Incorrect (will warn): 779 $string =~ /[:alpha:]/ 780 781The latter pattern would be a character class consisting of a colon, 782and the letters C<a>, C<l>, C<p> and C<h>. 783 784POSIX character classes can be part of a larger bracketed character class. 785For example, 786 787 [01[:alpha:]%] 788 789is valid and matches '0', '1', any alphabetic character, and the percent sign. 790 791Perl recognizes the following POSIX character classes: 792 793 alpha Any alphabetical character (e.g., [A-Za-z]). 794 alnum Any alphanumeric character (e.g., [A-Za-z0-9]). 795 ascii Any character in the ASCII character set. 796 blank A GNU extension, equal to a space or a horizontal tab ("\t"). 797 cntrl Any control character. See Note [2] below. 798 digit Any decimal digit (e.g., [0-9]), equivalent to "\d". 799 graph Any printable character, excluding a space. See Note [3] below. 800 lower Any lowercase character (e.g., [a-z]). 801 print Any printable character, including a space. See Note [4] below. 802 punct Any graphical character excluding "word" characters. Note [5]. 803 space Any whitespace character. "\s" including the vertical tab 804 ("\cK"). 805 upper Any uppercase character (e.g., [A-Z]). 806 word A Perl extension (e.g., [A-Za-z0-9_]), equivalent to "\w". 807 xdigit Any hexadecimal digit (e.g., [0-9a-fA-F]). Note [7]. 808 809Like the L<Unicode properties|/Unicode Properties>, most of the POSIX 810properties match the same regardless of whether case-insensitive (C</i>) 811matching is in effect or not. The two exceptions are C<[:upper:]> and 812C<[:lower:]>. Under C</i>, they each match the union of C<[:upper:]> and 813C<[:lower:]>. 814 815Most POSIX character classes have two Unicode-style C<\p> property 816counterparts. (They are not official Unicode properties, but Perl extensions 817derived from official Unicode properties.) The table below shows the relation 818between POSIX character classes and these counterparts. 819 820One counterpart, in the column labelled "ASCII-range Unicode" in 821the table, matches only characters in the ASCII character set. 822 823The other counterpart, in the column labelled "Full-range Unicode", matches any 824appropriate characters in the full Unicode character set. For example, 825C<\p{Alpha}> matches not just the ASCII alphabetic characters, but any 826character in the entire Unicode character set considered alphabetic. 827An entry in the column labelled "backslash sequence" is a (short) 828equivalent. 829 830 [[:...:]] ASCII-range Full-range backslash Note 831 Unicode Unicode sequence 832 ----------------------------------------------------- 833 alpha \p{PosixAlpha} \p{XPosixAlpha} 834 alnum \p{PosixAlnum} \p{XPosixAlnum} 835 ascii \p{ASCII} 836 blank \p{PosixBlank} \p{XPosixBlank} \h [1] 837 or \p{HorizSpace} [1] 838 cntrl \p{PosixCntrl} \p{XPosixCntrl} [2] 839 digit \p{PosixDigit} \p{XPosixDigit} \d 840 graph \p{PosixGraph} \p{XPosixGraph} [3] 841 lower \p{PosixLower} \p{XPosixLower} 842 print \p{PosixPrint} \p{XPosixPrint} [4] 843 punct \p{PosixPunct} \p{XPosixPunct} [5] 844 \p{PerlSpace} \p{XPerlSpace} \s [6] 845 space \p{PosixSpace} \p{XPosixSpace} [6] 846 upper \p{PosixUpper} \p{XPosixUpper} 847 word \p{PosixWord} \p{XPosixWord} \w 848 xdigit \p{PosixXDigit} \p{XPosixXDigit} [7] 849 850=over 4 851 852=item [1] 853 854C<\p{Blank}> and C<\p{HorizSpace}> are synonyms. 855 856=item [2] 857 858Control characters don't produce output as such, but instead usually control 859the terminal somehow: for example, newline and backspace are control characters. 860On ASCII platforms, in the ASCII range, characters whose code points are 861between 0 and 31 inclusive, plus 127 (C<DEL>) are control characters; on 862EBCDIC platforms, their counterparts are control characters. 863 864=item [3] 865 866Any character that is I<graphical>, that is, visible. This class consists 867of all alphanumeric characters and all punctuation characters. 868 869=item [4] 870 871All printable characters, which is the set of all graphical characters 872plus those whitespace characters which are not also controls. 873 874=item [5] 875 876C<\p{PosixPunct}> and C<[[:punct:]]> in the ASCII range match all 877non-controls, non-alphanumeric, non-space characters: 878C<[-!"#$%&'()*+,./:;<=E<gt>?@[\\\]^_`{|}~]> (although if a locale is in effect, 879it could alter the behavior of C<[[:punct:]]>). 880 881The similarly named property, C<\p{Punct}>, matches a somewhat different 882set in the ASCII range, namely 883C<[-!"#%&'()*,./:;?@[\\\]_{}]>. That is, it is missing the nine 884characters C<[$+E<lt>=E<gt>^`|~]>. 885This is because Unicode splits what POSIX considers to be punctuation into two 886categories, Punctuation and Symbols. 887 888C<\p{XPosixPunct}> and (under Unicode rules) C<[[:punct:]]>, match what 889C<\p{PosixPunct}> matches in the ASCII range, plus what C<\p{Punct}> 890matches. This is different than strictly matching according to 891C<\p{Punct}>. Another way to say it is that 892if Unicode rules are in effect, C<[[:punct:]]> matches all characters 893that Unicode considers punctuation, plus all ASCII-range characters that 894Unicode considers symbols. 895 896=item [6] 897 898C<\p{XPerlSpace}> and C<\p{Space}> match identically starting with Perl 899v5.18. In earlier versions, these differ only in that in non-locale 900matching, C<\p{XPerlSpace}> did not match the vertical tab, C<\cK>. 901Same for the two ASCII-only range forms. 902 903=item [7] 904 905Unlike C<[[:digit:]]> which matches digits in many writing systems, such 906as Thai and Devanagari, there are currently only two sets of hexadecimal 907digits, and it is unlikely that more will be added. This is because you 908not only need the ten digits, but also the six C<[A-F]> (and C<[a-f]>) 909to correspond. That means only the Latin script is suitable for these, 910and Unicode has only two sets of these, the familiar ASCII set, and the 911fullwidth forms starting at U+FF10 (FULLWIDTH DIGIT ZERO). 912 913=back 914 915There are various other synonyms that can be used besides the names 916listed in the table. For example, C<\p{XPosixAlpha}> can be written as 917C<\p{Alpha}>. All are listed in 918L<perluniprops/Properties accessible through \p{} and \P{}>. 919 920Both the C<\p> counterparts always assume Unicode rules are in effect. 921On ASCII platforms, this means they assume that the code points from 128 922to 255 are Latin-1, and that means that using them under locale rules is 923unwise unless the locale is guaranteed to be Latin-1 or UTF-8. In contrast, the 924POSIX character classes are useful under locale rules. They are 925affected by the actual rules in effect, as follows: 926 927=over 928 929=item If the C</a> modifier, is in effect ... 930 931Each of the POSIX classes matches exactly the same as their ASCII-range 932counterparts. 933 934=item otherwise ... 935 936=over 937 938=item For code points above 255 ... 939 940The POSIX class matches the same as its Full-range counterpart. 941 942=item For code points below 256 ... 943 944=over 945 946=item if locale rules are in effect ... 947 948The POSIX class matches according to the locale, except: 949 950=over 951 952=item C<word> 953 954also includes the platform's native underscore character, no matter what 955the locale is. 956 957=item C<ascii> 958 959on platforms that don't have the POSIX C<ascii> extension, this matches 960just the platform's native ASCII-range characters. 961 962=item C<blank> 963 964on platforms that don't have the POSIX C<blank> extension, this matches 965just the platform's native tab and space characters. 966 967=back 968 969=item if, instead, Unicode rules are in effect ... 970 971The POSIX class matches the same as the Full-range counterpart. 972 973=item otherwise ... 974 975The POSIX class matches the same as the ASCII range counterpart. 976 977=back 978 979=back 980 981=back 982 983Which rules apply are determined as described in 984L<perlre/Which character set modifier is in effect?>. 985 986=head4 Negation of POSIX character classes 987X<character class, negation> 988 989A Perl extension to the POSIX character class is the ability to 990negate it. This is done by prefixing the class name with a caret (C<^>). 991Some examples: 992 993 POSIX ASCII-range Full-range backslash 994 Unicode Unicode sequence 995 ----------------------------------------------------- 996 [[:^digit:]] \P{PosixDigit} \P{XPosixDigit} \D 997 [[:^space:]] \P{PosixSpace} \P{XPosixSpace} 998 \P{PerlSpace} \P{XPerlSpace} \S 999 [[:^word:]] \P{PerlWord} \P{XPosixWord} \W 1000 1001The backslash sequence can mean either ASCII- or Full-range Unicode, 1002depending on various factors as described in L<perlre/Which character set modifier is in effect?>. 1003 1004=head4 [= =] and [. .] 1005 1006Perl recognizes the POSIX character classes C<[=class=]> and 1007C<[.class.]>, but does not (yet?) support them. Any attempt to use 1008either construct raises an exception. 1009 1010=head4 Examples 1011 1012 /[[:digit:]]/ # Matches a character that is a digit. 1013 /[01[:lower:]]/ # Matches a character that is either a 1014 # lowercase letter, or '0' or '1'. 1015 /[[:digit:][:^xdigit:]]/ # Matches a character that can be anything 1016 # except the letters 'a' to 'f' and 'A' to 1017 # 'F'. This is because the main character 1018 # class is composed of two POSIX character 1019 # classes that are ORed together, one that 1020 # matches any digit, and the other that 1021 # matches anything that isn't a hex digit. 1022 # The OR adds the digits, leaving only the 1023 # letters 'a' to 'f' and 'A' to 'F' excluded. 1024 1025=head3 Extended Bracketed Character Classes 1026X<character class> 1027X<set operations> 1028 1029This is a fancy bracketed character class that can be used for more 1030readable and less error-prone classes, and to perform set operations, 1031such as intersection. An example is 1032 1033 /(?[ \p{Thai} & \p{Digit} ])/ 1034 1035This will match all the digit characters that are in the Thai script. 1036 1037This is an experimental feature available starting in 5.18, and is 1038subject to change as we gain field experience with it. Any attempt to 1039use it will raise a warning, unless disabled via 1040 1041 no warnings "experimental::regex_sets"; 1042 1043Comments on this feature are welcome; send email to 1044C<perl5-porters@perl.org>. 1045 1046The rules used by L<C<use re 'strict>|re/'strict' mode> apply to this 1047construct. 1048 1049We can extend the example above: 1050 1051 /(?[ ( \p{Thai} + \p{Lao} ) & \p{Digit} ])/ 1052 1053This matches digits that are in either the Thai or Laotian scripts. 1054 1055Notice the white space in these examples. This construct always has 1056the C<E<sol>xx> modifier turned on within it. 1057 1058The available binary operators are: 1059 1060 & intersection 1061 + union 1062 | another name for '+', hence means union 1063 - subtraction (the result matches the set consisting of those 1064 code points matched by the first operand, excluding any that 1065 are also matched by the second operand) 1066 ^ symmetric difference (the union minus the intersection). This 1067 is like an exclusive or, in that the result is the set of code 1068 points that are matched by either, but not both, of the 1069 operands. 1070 1071There is one unary operator: 1072 1073 ! complement 1074 1075All the binary operators left associate; C<"&"> is higher precedence 1076than the others, which all have equal precedence. The unary operator 1077right associates, and has highest precedence. Thus this follows the 1078normal Perl precedence rules for logical operators. Use parentheses to 1079override the default precedence and associativity. 1080 1081The main restriction is that everything is a metacharacter. Thus, 1082you cannot refer to single characters by doing something like this: 1083 1084 /(?[ a + b ])/ # Syntax error! 1085 1086The easiest way to specify an individual typable character is to enclose 1087it in brackets: 1088 1089 /(?[ [a] + [b] ])/ 1090 1091(This is the same thing as C<[ab]>.) You could also have said the 1092equivalent: 1093 1094 /(?[[ a b ]])/ 1095 1096(You can, of course, specify single characters by using, C<\x{...}>, 1097C<\N{...}>, etc.) 1098 1099This last example shows the use of this construct to specify an ordinary 1100bracketed character class without additional set operations. Note the 1101white space within it. This is allowed because C<E<sol>xx> is 1102automatically turned on within this construct. 1103 1104All the other escapes accepted by normal bracketed character classes are 1105accepted here as well. 1106 1107Because this construct compiles under 1108L<C<use re 'strict>|re/'strict' mode>, unrecognized escapes that 1109generate warnings in normal classes are fatal errors here, as well as 1110all other warnings from these class elements, as well as some 1111practices that don't currently warn outside C<re 'strict'>. For example 1112you cannot say 1113 1114 /(?[ [ \xF ] ])/ # Syntax error! 1115 1116You have to have two hex digits after a braceless C<\x> (use a leading 1117zero to make two). These restrictions are to lower the incidence of 1118typos causing the class to not match what you thought it would. 1119 1120If a regular bracketed character class contains a C<\p{}> or C<\P{}> and 1121is matched against a non-Unicode code point, a warning may be 1122raised, as the result is not Unicode-defined. No such warning will come 1123when using this extended form. 1124 1125The final difference between regular bracketed character classes and 1126these, is that it is not possible to get these to match a 1127multi-character fold. Thus, 1128 1129 /(?[ [\xDF] ])/iu 1130 1131does not match the string C<ss>. 1132 1133You don't have to enclose POSIX class names inside double brackets, 1134hence both of the following work: 1135 1136 /(?[ [:word:] - [:lower:] ])/ 1137 /(?[ [[:word:]] - [[:lower:]] ])/ 1138 1139Any contained POSIX character classes, including things like C<\w> and C<\D> 1140respect the C<E<sol>a> (and C<E<sol>aa>) modifiers. 1141 1142Note that C<< (?[ ]) >> is a regex-compile-time construct. Any attempt 1143to use something which isn't knowable at the time the containing regular 1144expression is compiled is a fatal error. In practice, this means 1145just three limitations: 1146 1147=over 4 1148 1149=item 1 1150 1151When compiled within the scope of C<use locale> (or the C<E<sol>l> regex 1152modifier), this construct assumes that the execution-time locale will be 1153a UTF-8 one, and the generated pattern always uses Unicode rules. What 1154gets matched or not thus isn't dependent on the actual runtime locale, so 1155tainting is not enabled. But a C<locale> category warning is raised 1156if the runtime locale turns out to not be UTF-8. 1157 1158=item 2 1159 1160Any 1161L<user-defined property|perlunicode/"User-Defined Character Properties"> 1162used must be already defined by the time the regular expression is 1163compiled (but note that this construct can be used instead of such 1164properties). 1165 1166=item 3 1167 1168A regular expression that otherwise would compile 1169using C<E<sol>d> rules, and which uses this construct will instead 1170use C<E<sol>u>. Thus this construct tells Perl that you don't want 1171C<E<sol>d> rules for the entire regular expression containing it. 1172 1173=back 1174 1175Note that skipping white space applies only to the interior of this 1176construct. There must not be any space between any of the characters 1177that form the initial C<(?[>. Nor may there be space between the 1178closing C<])> characters. 1179 1180Just as in all regular expressions, the pattern can be built up by 1181including variables that are interpolated at regex compilation time. 1182But its best to compile each sub-component. 1183 1184 my $thai_or_lao = qr/(?[ \p{Thai} + \p{Lao} ])/; 1185 my $lower = qr/(?[ \p{Lower} + \p{Digit} ])/; 1186 1187When these are embedded in another pattern, what they match does not 1188change, regardless of parenthesization or what modifiers are in effect 1189in that outer pattern. If you fail to compile the subcomponents, you 1190can get some nasty surprises. For example: 1191 1192 my $thai_or_lao = '\p{Thai} + \p{Lao}'; 1193 ... 1194 qr/(?[ \p{Digit} & $thai_or_lao ])/; 1195 1196compiles to 1197 1198 qr/(?[ \p{Digit} & \p{Thai} + \p{Lao} ])/; 1199 1200But this does not have the effect that someone reading the source code 1201would likely expect, as the intersection applies just to C<\p{Thai}>, 1202excluding the Laotian. Its best to compile the subcomponents, but you 1203could also parenthesize the component pieces: 1204 1205 my $thai_or_lao = '( \p{Thai} + \p{Lao} )'; 1206 1207But any modifiers will still apply to all the components: 1208 1209 my $lower = '\p{Lower} + \p{Digit}'; 1210 qr/(?[ \p{Greek} & $lower ])/i; 1211 1212matches upper case things. So just, compile the subcomponents, as 1213illustrated above. 1214 1215Due to the way that Perl parses things, your parentheses and brackets 1216may need to be balanced, even including comments. If you run into any 1217examples, please submit them to L<https://github.com/Perl/perl5/issues>, 1218so that we can have a concrete example for this man page. 1219 1220We may change it so that things that remain legal uses in normal bracketed 1221character classes might become illegal within this experimental 1222construct. One proposal, for example, is to forbid adjacent uses of the 1223same character, as in C<(?[ [aa] ])>. The motivation for such a change 1224is that this usage is likely a typo, as the second "a" adds nothing. 1225