NAME
perlrecharclass - Perl Regular Expression Character Classes
DESCRIPTION
The top level documentation about Perl regular expressions is found in perlre.
This manual page discusses the syntax and use of character classes in Perl regular expressions.
A character class is a way of denoting a set of characters in such a way that one character of the set is matched. It's important to remember that: matching a character class consumes exactly one character in the source string. (The source string is the string the regular expression is matched against.)
There are three types of character classes in Perl regular expressions: the dot, backslash sequences, and the form enclosed in square brackets. Keep in mind, though, that often the term "character class" is used to mean just the bracketed form. Certainly, most Perl documentation does that.
The dot
The dot (or period), . is probably the most used, and certainly the most well-known character class. By default, a dot matches any character, except for the newline. The default can be changed to add matching the newline by using the single line modifier: either for the entire regular expression with the /s modifier, or locally with (?s). (The experimental \N backslash sequence, described below, matches any character except newline without regard to the single line modifier.)
Here are some examples:
"a" =~ /./ # Match
"." =~ /./ # Match
"" =~ /./ # No match (dot has to match a character)
"\n" =~ /./ # No match (dot does not match a newline)
"\n" =~ /./s # Match (global 'single line' modifier)
"\n" =~ /(?s:.)/ # Match (local 'single line' modifier)
"ab" =~ /^.$/ # No match (dot matches one character)Backslash sequences
A backslash sequence is a sequence of characters, the first one of which is a backslash. Perl ascribes special meaning to many such sequences, and some of these are character classes. That is, they match a single character each, provided that the character belongs to the specific set of characters defined by the sequence.
Here's a list of the backslash sequences that are character classes. They are discussed in more detail below. (For the backslash sequences that aren't character classes, see perlrebackslash.)
\d Match a decimal digit character.
\D Match a non-decimal-digit character.
\w Match a "word" character.
\W Match a non-"word" character.
\s Match a whitespace character.
\S Match a non-whitespace character.
\h Match a horizontal whitespace character.
\H Match a character that isn't horizontal whitespace.
\v Match a vertical whitespace character.
\V Match a character that isn't vertical whitespace.
\N Match a character that isn't a newline. Experimental.
\pP, \p{Prop} Match a character that has the given Unicode property.
\PP, \P{Prop} Match a character that doesn't have the Unicode propertyDigits
\d matches a single character considered to be a decimal digit. What is considered a decimal digit depends on several factors, detailed below in "Locale, EBCDIC, Unicode and UTF-8". If those factors indicate a Unicode interpretation, \d not only matches the digits '0' - '9', but also Arabic, Devanagari, and digits from other languages. Otherwise, if a locale is in effect, it matches whatever characters that locale considers decimal digits. Only when neither a Unicode interpretation nor locale prevails does \d match only the digits '0' to '9' alone.
Unicode digits may cause some confusion, and some security issues. In UTF-8 strings, unless the "a" regular expression modifier is specified, \d matches the same characters matched by \p{General_Category=Decimal_Number}, or synonymously, \p{General_Category=Digit}. Starting with Unicode version 4.1, this is the same set of characters matched by \p{Numeric_Type=Decimal}.
But Unicode also has a different property with a similar name, \p{Numeric_Type=Digit}, which matches a completely different set of characters. These characters are things such as subscripts.
The design intent is for \d to match all digits (and no other characters) that can be used with "normal" big-endian positional decimal syntax, whereby a sequence of such digits {N0, N1, N2, ...Nn} has the numeric value (...(N0 * 10 + N1) * 10 + N2) * 10 ... + Nn). In Unicode 5.2, the Tamil digits (U+0BE6 - U+0BEF) can also legally be used in old-style Tamil numbers in which they would appear no more than one in a row, separated by characters that mean "times 10", "times 100", etc. (See http://www.unicode.org/notes/tn21.)
Some non-European digits that \d matches look like European ones, but have different values. For example, BENGALI DIGIT FOUR (U+09EA) looks very much like an ASCII DIGIT EIGHT (U+0038).
It may be useful for security purposes for an application to require that all digits in a row be from the same script. See "charscript()" in Unicode::UCD.
Any character not matched by \d is matched by \D.
Word characters
A \w matches a single alphanumeric character (an alphabetic character, or a decimal digit) or a connecting punctuation character, such as an underscore ("_"). It does not match a whole word. To match a whole word, use \w+. This isn't the same thing as matching an English word, but in the ASCII range it is the same as a string of Perl-identifier characters. What is considered a word character depends on several factors, detailed below in "Locale, EBCDIC, Unicode and UTF-8". If those factors indicate a Unicode interpretation, \w matches the characters considered word characters in the Unicode database. That is, it not only matches ASCII letters, but also Thai letters, Greek letters, etc. This includes connector punctuation (like the underscore) which connect two words together, or diacritics, such as a COMBINING TILDE and the modifier letters, which are generally used to add auxiliary markings to letters. If a Unicode interpretation is not indicated, \w matches those characters considered word characters by the current locale or EBCDIC code page. Without a locale or EBCDIC code page, \w matches the underscore and ASCII letters and digits.
There are a number of security issues with the full Unicode list of word characters. See http://unicode.org/reports/tr36.
Also, for a somewhat finer-grained set of characters that are in programming language identifiers beyond the ASCII range, you may wish to instead use the more customized Unicode properties, "ID_Start", ID_Continue", "XID_Start", and "XID_Continue". See http://unicode.org/reports/tr31.
Any character not matched by \w is matched by \W.
Whitespace
\s matches any single character considered whitespace. The exact set of characters matched by \s depends on several factors, detailed below in "Locale, EBCDIC, Unicode and UTF-8". If those factors indicate a Unicode interpretation, \s matches what is considered whitespace in the Unicode database; the complete list is in the table below. Otherwise, if a locale or EBCDIC code page is in effect, \s matches whatever is considered whitespace by the current locale or EBCDIC code page. Without a locale or EBCDIC code page, \s matches the horizontal tab (\t), the newline (\n), the form feed (\f), the carriage return (\r), and the space. (Note that it doesn't match the vertical tab, \cK.) Perhaps the most notable possible surprise is that \s matches a non-breaking space only if a Unicode interpretation is indicated, or the locale or EBCDIC code page that is in effect happens to have that character.
Any character not matched by \s is matched by \S.
\h matches any character considered horizontal whitespace; this includes the space and tab characters and several others listed in the table below. \H matches any character not considered horizontal whitespace.
\v matches any character considered vertical whitespace; this includes the carriage return and line feed characters (newline) plus several other characters, all listed in the table below. \V matches any character not considered vertical whitespace.
\R matches anything that can be considered a newline under Unicode rules. It's not a character class, as it can match a multi-character sequence. Therefore, it cannot be used inside a bracketed character class; use \v instead (vertical whitespace). Details are discussed in perlrebackslash.
Note that unlike \s, \d and \w, \h and \v always match the same characters, without regard to other factors, such as whether the source string is in UTF-8 format.
One might think that \s is equivalent to [\h\v]. This is not true. The vertical tab ("\x0b") is not matched by \s, it is however considered vertical whitespace. Furthermore, if the source string is not in UTF-8 format, and any locale or EBCDIC code page that is in effect doesn't include them, the next line (ASCII-platform "\x85") and the no-break space (ASCII-platform "\xA0") characters are not matched by \s, but are by \v and \h respectively. If the "a" modifier is not in effect and the source string is in UTF-8 format, both the next line and the no-break space are matched by \s.
The following table is a complete listing of characters matched by \s, \h and \v as of Unicode 5.2.
The first column gives the code point of the character (in hex format), the second column gives the (Unicode) name. The third column indicates by which class(es) the character is matched (assuming no locale or EBCDIC code page is in effect that changes the \s matching).
0x00009 CHARACTER TABULATION h s
0x0000a LINE FEED (LF) vs
0x0000b LINE TABULATION v
0x0000c FORM FEED (FF) vs
0x0000d CARRIAGE RETURN (CR) vs
0x00020 SPACE h s
0x00085 NEXT LINE (NEL) vs [1]
0x000a0 NO-BREAK SPACE h s [1]
0x01680 OGHAM SPACE MARK h s
0x0180e MONGOLIAN VOWEL SEPARATOR h s
0x02000 EN QUAD h s
0x02001 EM QUAD h s
0x02002 EN SPACE h s
0x02003 EM SPACE h s
0x02004 THREE-PER-EM SPACE h s
0x02005 FOUR-PER-EM SPACE h s
0x02006 SIX-PER-EM SPACE h s
0x02007 FIGURE SPACE h s
0x02008 PUNCTUATION SPACE h s
0x02009 THIN SPACE h s
0x0200a HAIR SPACE h s
0x02028 LINE SEPARATOR vs
0x02029 PARAGRAPH SEPARATOR vs
0x0202f NARROW NO-BREAK SPACE h s
0x0205f MEDIUM MATHEMATICAL SPACE h s
0x03000 IDEOGRAPHIC SPACE h s- [1]
-
NEXT LINE and NO-BREAK SPACE only match
\sif the source string is in UTF-8 format and the"a"modifier is not in effect, or if the locale or EBCDIC code page in effect includes them.
It is worth noting that \d, \w, etc, match single characters, not complete numbers or words. To match a number (that consists of digits), use \d+; to match a word, use \w+.
\N
\N is new in 5.12, and is experimental. It, like the dot, matches any character that is not a newline. The difference is that \N is not influenced by the single line regular expression modifier (see "The dot" above). Note that the form \N{...} may mean something completely different. When the {...} is a quantifier, it means to match a non-newline character that many times. For example, \N{3} means to match 3 non-newlines; \N{5,} means to match 5 or more non-newlines. But if {...} is not a legal quantifier, it is presumed to be a named character. See charnames for those. For example, none of \N{COLON}, \N{4F}, and \N{F4} contain legal quantifiers, so Perl will try to find characters whose names are respectively COLON, 4F, and F4.
Unicode Properties
\pP and \p{Prop} are character classes to match characters that fit given Unicode properties. One letter property names can be used in the \pP form, with the property name following the \p, otherwise, braces are required. When using braces, there is a single form, which is just the property name enclosed in the braces, and a compound form which looks like \p{name=value}, which means to match if the property "name" for the character has that particular "value". For instance, a match for a number can be written as /\pN/ or as /\p{Number}/, or as /\p{Number=True}/. Lowercase letters are matched by the property Lowercase_Letter which has as short form Ll. They need the braces, so are written as /\p{Ll}/ or /\p{Lowercase_Letter}/, or /\p{General_Category=Lowercase_Letter}/ (the underscores are optional). /\pLl/ is valid, but means something different. It matches a two character string: a letter (Unicode property \pL), followed by a lowercase l.
Note that almost all properties are immune to case-insensitive matching. That is, adding a /i regular expression modifier does not change what they match. There are two sets affected. The first set is Uppercase_Letter, Lowercase_Letter, and Titlecase_Letter, all of which match Cased_Letter under /i matching. The second set is Uppercase, Lowercase, and Titlecase, all of which match Cased under /i matching. (The difference between these sets is that some things, such as Roman Numerals, come in both upper and lower case so they are Cased, but aren't considered to be letters, so they aren't Cased_Letters. They're actually Letter_Numbers.) This set also includes its subsets PosixUpper and PosixLower, both of which under /i matching match PosixAlpha.
For more details on Unicode properties, see "Unicode Character Properties" in perlunicode; for a complete list of possible properties, see "Properties accessible through \p{} and \P{}" in perluniprops, which notes all forms that have /i differences. It is also possible to define your own properties. This is discussed in "User-Defined Character Properties" in perlunicode.
Examples
"a" =~ /\w/ # Match, "a" is a 'word' character.
"7" =~ /\w/ # Match, "7" is a 'word' character as well.
"a" =~ /\d/ # No match, "a" isn't a digit.
"7" =~ /\d/ # Match, "7" is a digit.
" " =~ /\s/ # Match, a space is whitespace.
"a" =~ /\D/ # Match, "a" is a non-digit.
"7" =~ /\D/ # No match, "7" is not a non-digit.
" " =~ /\S/ # No match, a space is not non-whitespace.
" " =~ /\h/ # Match, space is horizontal whitespace.
" " =~ /\v/ # No match, space is not vertical whitespace.
"\r" =~ /\v/ # Match, a return is vertical whitespace.
"a" =~ /\pL/ # Match, "a" is a letter.
"a" =~ /\p{Lu}/ # No match, /\p{Lu}/ matches upper case letters.
"\x{0e0b}" =~ /\p{Thai}/ # Match, \x{0e0b} is the character
# 'THAI CHARACTER SO SO', and that's in
# Thai Unicode class.
"a" =~ /\P{Lao}/ # Match, as "a" is not a Laotian character.Bracketed Character Classes
The third form of character class you can use in Perl regular expressions is the bracketed character class. In its simplest form, it lists the characters that may be matched, surrounded by square brackets, like this: [aeiou]. This matches one of a, e, i, o or u. Like the other character classes, exactly one character is matched. To match a longer string consisting of characters mentioned in the character class, follow the character class with a quantifier. For instance, [aeiou]+ matches one or more lowercase English vowels.
Repeating a character in a character class has no effect; it's considered to be in the set only once.
Examples:
"e" =~ /[aeiou]/ # Match, as "e" is listed in the class.
"p" =~ /[aeiou]/ # No match, "p" is not listed in the class.
"ae" =~ /^[aeiou]$/ # No match, a character class only matches
# a single character.
"ae" =~ /^[aeiou]+$/ # Match, due to the quantifier.Special Characters Inside a Bracketed Character Class
Most characters that are meta characters in regular expressions (that is, characters that carry a special meaning like ., *, or () lose their special meaning and can be used inside a character class without the need to escape them. For instance, [()] matches either an opening parenthesis, or a closing parenthesis, and the parens inside the character class don't group or capture.
Characters that may carry a special meaning inside a character class are: \, ^, -, [ and ], and are discussed below. They can be escaped with a backslash, although this is sometimes not needed, in which case the backslash may be omitted.
The sequence \b is special inside a bracketed character class. While outside the character class, \b is an assertion indicating a point that does not have either two word characters or two non-word characters on either side, inside a bracketed character class, \b matches a backspace character.
The sequences \a, \c, \e, \f, \n, \N{NAME}, \N{U+hex char}, \r, \t, and \x are also special and have the same meanings as they do outside a bracketed character class. (However, inside a bracketed character class, if \N{NAME} expands to a sequence of characters, only the first one in the sequence is used, with a warning.)
Also, a backslash followed by two or three octal digits is considered an octal number.
A [ is not special inside a character class, unless it's the start of a POSIX character class (see "POSIX Character Classes" below). It normally does not need escaping.
A ] is normally either the end of a POSIX character class (see "POSIX Character Classes" below), or it signals the end of the bracketed character class. If you want to include a ] in the set of characters, you must generally escape it.
However, if the ] is the first (or the second if the first character is a caret) character of a bracketed character class, it does not denote the end of the class (as you cannot have an empty class) and is considered part of the set of characters that can be matched without escaping.
Examples:
"+" =~ /[+?*]/ # Match, "+" in a character class is not special.
"\cH" =~ /[\b]/ # Match, \b inside in a character class
# is equivalent to a backspace.
"]" =~ /[][]/ # Match, as the character class contains.
# both [ and ].
"[]" =~ /[[]]/ # Match, the pattern contains a character class
# containing just ], and the character class is
# followed by a ].Character Ranges
It is not uncommon to want to match a range of characters. Luckily, instead of listing all characters in the range, one may use the hyphen (-). If inside a bracketed character class you have two characters separated by a hyphen, it's treated as if all characters between the two were in the class. For instance, [0-9] matches any ASCII digit, and [a-m] matches any lowercase letter from the first half of the old ASCII alphabet.
Note that the two characters on either side of the hyphen are not necessarily both letters or both digits. Any character is possible, although not advisable. ['-?] contains a range of characters, but most people will not know which characters that means. Furthermore, such ranges may lead to portability problems if the code has to run on a platform that uses a different character set, such as EBCDIC.
If a hyphen in a character class cannot syntactically be part of a range, for instance because it is the first or the last character of the character class, or if it immediately follows a range, the hyphen isn't special, and so is considered a character to be matched literally. If you want a hyphen in your set of characters to be matched and its position in the class is such that it could be considered part of a range, you must escape that hyphen with a backslash.
Examples:
[a-z] # Matches a character that is a lower case ASCII letter.
[a-fz] # Matches any letter between 'a' and 'f' (inclusive) or
# the letter 'z'.
[-z] # Matches either a hyphen ('-') or the letter 'z'.
[a-f-m] # Matches any letter between 'a' and 'f' (inclusive), the
# hyphen ('-'), or the letter 'm'.
['-?] # Matches any of the characters '()*+,-./0123456789:;<=>?
# (But not on an EBCDIC platform).Negation
It is also possible to instead list the characters you do not want to match. You can do so by using a caret (^) as the first character in the character class. For instance, [^a-z] matches any character that is not a lowercase ASCII letter, which therefore includes almost a hundred thousand Unicode letters.
This syntax make the caret a special character inside a bracketed character class, but only if it is the first character of the class. So if you want the caret as one of the characters to match, either escape the caret or else not list it first.
Examples:
"e" =~ /[^aeiou]/ # No match, the 'e' is listed.
"x" =~ /[^aeiou]/ # Match, as 'x' isn't a lowercase vowel.
"^" =~ /[^^]/ # No match, matches anything that isn't a caret.
"^" =~ /[x^]/ # Match, caret is not special here.Backslash Sequences
You can put any backslash sequence character class (with the exception of \N and \R) inside a bracketed character class, and it will act just as if you had put all characters matched by the backslash sequence inside the character class. For instance, [a-f\d] matches any decimal digit, or any of the lowercase letters between 'a' and 'f' inclusive.
\N within a bracketed character class must be of the forms \N{name} or \N{U+hex char}, and NOT be the form that matches non-newlines, for the same reason that a dot . inside a bracketed character class loses its special meaning: it matches nearly anything, which generally isn't what you want to happen.
Examples:
/[\p{Thai}\d]/ # Matches a character that is either a Thai
# character, or a digit.
/[^\p{Arabic}()]/ # Matches a character that is neither an Arabic
# character, nor a parenthesis.Backslash sequence character classes cannot form one of the endpoints of a range. Thus, you can't say:
/[\p{Thai}-\d]/ # Wrong!POSIX Character Classes
POSIX character classes have the form [:class:], where class is name, and the [: and :] delimiters. POSIX character classes only appear inside bracketed character classes, and are a convenient and descriptive way of listing a group of characters, though they can suffer from portability issues (see below and "Locale, EBCDIC, Unicode and UTF-8").
Be careful about the syntax,
# Correct:
$string =~ /[[:alpha:]]/
# Incorrect (will warn):
$string =~ /[:alpha:]/The latter pattern would be a character class consisting of a colon, and the letters a, l, p and h. POSIX character classes can be part of a larger bracketed character class. For example,
[01[:alpha:]%]is valid and matches '0', '1', any alphabetic character, and the percent sign.
Perl recognizes the following POSIX character classes:
alpha Any alphabetical character ("[A-Za-z]").
alnum Any alphanumeric character. ("[A-Za-z0-9]")
ascii Any character in the ASCII character set.
blank A GNU extension, equal to a space or a horizontal tab ("\t").
cntrl Any control character. See Note [2] below.
digit Any decimal digit ("[0-9]"), equivalent to "\d".
graph Any printable character, excluding a space. See Note [3] below.
lower Any lowercase character ("[a-z]").
print Any printable character, including a space. See Note [4] below.
punct Any graphical character excluding "word" characters. Note [5].
space Any whitespace character. "\s" plus the vertical tab ("\cK").
upper Any uppercase character ("[A-Z]").
word A Perl extension ("[A-Za-z0-9_]"), equivalent to "\w".
xdigit Any hexadecimal digit ("[0-9a-fA-F]").Most POSIX character classes have two Unicode-style \p property counterparts. (They are not official Unicode properties, but Perl extensions derived from official Unicode properties.) The table below shows the relation between POSIX character classes and these counterparts.
One counterpart, in the column labelled "ASCII-range Unicode" in the table, matches only characters in the ASCII character set.
The other counterpart, in the column labelled "Full-range Unicode", matches any appropriate characters in the full Unicode character set. For example, \p{Alpha} matches not just the ASCII alphabetic characters, but any character in the entire Unicode character set considered alphabetic. The column labelled "backslash sequence" is a (short) synonym for the Full-range Unicode form.
(Each of the counterparts has various synonyms as well. "Properties accessible through \p{} and \P{}" in perluniprops lists all synonyms, plus all characters matched by each ASCII-range property. For example, \p{AHex} is a synonym for \p{ASCII_Hex_Digit}, and any \p property name can be prefixed with "Is" such as \p{IsAlpha}.)
Both the \p forms are unaffected by any locale in effect, or whether the string is in UTF-8 format or not, or whether the platform is EBCDIC or not. In contrast, the POSIX character classes are affected, unless the regular expression is compiled with the "a" modifier. If the "a" modifier is not in effect, and the source string is in UTF-8 format, the POSIX classes behave like their "Full-range" Unicode counterparts. If "a" modifier is in effect; or the source string is not in UTF-8 format, and no locale is in effect, and the platform is not EBCDIC, all the POSIX classes behave like their ASCII-range counterparts. Otherwise, they behave based on the rules of the locale or EBCDIC code page.
It is proposed to change this behavior in a future release of Perl so that the the UTF-8-ness of the source string will be irrelevant to the behavior of the POSIX character classes. This means they will always behave in strict accordance with the official POSIX standard. That is, if either locale or EBCDIC code page is present, they will behave in accordance with those; if absent, the classes will match only their ASCII-range counterparts. If you wish to comment on this proposal, send email to perl5-porters@perl.org.
[[:...:]] ASCII-range Full-range backslash Note
Unicode Unicode sequence
-----------------------------------------------------
alpha \p{PosixAlpha} \p{XPosixAlpha}
alnum \p{PosixAlnum} \p{XPosixAlnum}
ascii \p{ASCII}
blank \p{PosixBlank} \p{XPosixBlank} \h [1]
or \p{HorizSpace} [1]
cntrl \p{PosixCntrl} \p{XPosixCntrl} [2]
digit \p{PosixDigit} \p{XPosixDigit} \d
graph \p{PosixGraph} \p{XPosixGraph} [3]
lower \p{PosixLower} \p{XPosixLower}
print \p{PosixPrint} \p{XPosixPrint} [4]
punct \p{PosixPunct} \p{XPosixPunct} [5]
\p{PerlSpace} \p{XPerlSpace} \s [6]
space \p{PosixSpace} \p{XPosixSpace} [6]
upper \p{PosixUpper} \p{XPosixUpper}
word \p{PosixWord} \p{XPosixWord} \w
xdigit \p{ASCII_Hex_Digit} \p{XPosixXDigit}- [1]
-
\p{Blank}and\p{HorizSpace}are synonyms. - [2]
-
Control characters don't produce output as such, but instead usually control the terminal somehow: for example, newline and backspace are control characters. In the ASCII range, characters whose ordinals are between 0 and 31 inclusive, plus 127 (
DEL) are control characters.On EBCDIC platforms, it is likely that the code page will define
[[:cntrl:]]to be the EBCDIC equivalents of the ASCII controls, plus the controls that in Unicode have ordinals from 128 through 159. - [3]
-
Any character that is graphical, that is, visible. This class consists of all alphanumeric characters and all punctuation characters.
- [4]
-
All printable characters, which is the set of all graphical characters plus those whitespace characters which are not also controls.
- [5]
-
\p{PosixPunct}and[[:punct:]]in the ASCII range match all non-controls, non-alphanumeric, non-space characters:[-!"#$%&'()*+,./:;<=>?@[\\\]^_`{|}~](although if a locale is in effect, it could alter the behavior of[[:punct:]]).The similarly named property,
\p{Punct}, matches a somewhat different set in the ASCII range, namely[-!"#%&'()*,./:;?@[\\\]_{}]. That is, it is missing[$+<=>^`|~]. This is because Unicode splits what POSIX considers to be punctuation into two categories, Punctuation and Symbols.\p{XPosixPunct}and (in Unicode mode)[[:punct:]], match what\p{PosixPunct}matches in the ASCII range, plus what\p{Punct}matches. This is different than strictly matching according to\p{Punct}. Another way to say it is that for a UTF-8 string,[[:punct:]]matches all characters that Unicode considers punctuation, plus all ASCII-range characters that Unicode considers symbols. - [6]
-
\p{SpacePerl}and\p{Space}differ only in that\p{Space}additionally matches the vertical tab,\cK. Same for the two ASCII-only range forms.
There are various other synonyms that can be used for these besides \p{HorizSpace} and \\p{XPosixBlank}. For example, \p{PosixAlpha} can be written as \p{Alpha}. All are listed in "Properties accessible through \p{} and \P{}" in perluniprops.
Negation
A Perl extension to the POSIX character class is the ability to negate it. This is done by prefixing the class name with a caret (^). Some examples:
POSIX ASCII-range Full-range backslash
Unicode Unicode sequence
-----------------------------------------------------
[[:^digit:]] \P{PosixDigit} \P{XPosixDigit} \D
[[:^space:]] \P{PosixSpace} \P{XPosixSpace}
\P{PerlSpace} \P{XPerlSpace} \S
[[:^word:]] \P{PerlWord} \P{XPosixWord} \WThe backslash sequence can mean either ASCII- or Full-range Unicode, depending on various factors. See "Locale, EBCDIC, Unicode and UTF-8" below.
[= =] and [. .]
Perl recognizes the POSIX character classes [=class=] and [.class.], but does not (yet?) support them. Any attempt to use either construct raises an exception.
Examples
/[[:digit:]]/ # Matches a character that is a digit.
/[01[:lower:]]/ # Matches a character that is either a
# lowercase letter, or '0' or '1'.
/[[:digit:][:^xdigit:]]/ # Matches a character that can be anything
# except the letters 'a' to 'f'. This is
# because the main character class is composed
# of two POSIX character classes that are ORed
# together, one that matches any digit, and
# the other that matches anything that isn't a
# hex digit. The result matches all
# characters except the letters 'a' to 'f' and
# 'A' to 'F'.Locale, EBCDIC, Unicode and UTF-8
Some of the character classes have a somewhat different behaviour depending on the internal encoding of the source string, whether the regular expression is marked as having Unicode semantics, whatever locale is in effect, and whether the program is running on an EBCDIC platform.
\w, \d, \s and the POSIX character classes (and their negations, including \W, \D, \S) have this behaviour. (Since the backslash sequences \b and \B are defined in terms of \w and \W, they also are affected.)
Starting in Perl 5.14, if the regular expression is compiled with the "a" modifier, the behavior doesn't differ regardless of any other factors. \d matches the 10 digits 0-9; \D any character but those 10; \s, exactly the five characters "[ \f\n\r\t]"; \w only the 63 characters "[A-Za-z0-9_]"; and the "[[:posix:]]" classes only the appropriate ASCII characters, the same characters as are matched by the corresponding \p{} property given in the "ASCII-range Unicode" column in the table above. (The behavior of all of their complements follows the same paradigm.)
Otherwise, a regular expression is marked for Unicode semantics if it is encoded in utf8 (usually as a result of including a literal character whose code point is above 255), or if it contains a \N{U+...} or \N{name} construct, or (starting in Perl 5.14) if it was compiled in the scope of a use feature "unicode_strings" pragma and not in the scope of a use locale pragma, or has the "u" regular expression modifier.
Note that one can specify "use re '/l'" for example, for any regular expression modifier, and this has precedence over either of the use feature "unicode_strings" or use locale pragmas.
The differences in behavior between locale and non-locale semantics can affect any character whose code point is 255 or less. The differences in behavior between Unicode and non-Unicode semantics affects only ASCII platforms, and only when matching against characters whose code points are between 128 and 255 inclusive. See "The "Unicode Bug"" in perlunicode.
For portability reasons, unless the "a" modifier is specified, it may be better to not use \w, \d, \s or the POSIX character classes and use the Unicode properties instead.
That way you can control whether you want matching of characters in the ASCII character set alone, or whether to match Unicode characters. use feature "unicode_strings" allows seamless Unicode behavior no matter the internal encodings, but won't allow restricting to ASCII characters only.
Examples
$str = "\xDF"; # $str is not in UTF-8 format.
$str =~ /^\w/; # No match, as $str isn't in UTF-8 format.
$str .= "\x{0e0b}"; # Now $str is in UTF-8 format.
$str =~ /^\w/; # Match! $str is now in UTF-8 format.
chop $str;
$str =~ /^\w/; # Still a match! $str remains in UTF-8 format.