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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 property

       Digits

       "\d" matches a single character considered to be a decimal digit.  If the "/a" modifier in
       effect, it matches [0-9].  Otherwise, it matches anything that is matched by "\p{Digit}",
       which includes [0-9].  (An unlikely possible exception is that under locale matching
       rules, the current locale might not have [0-9] matched by "\d", and/or might match other
       characters whose code point is less than 256.  Such a locale definition would be in
       violation of the C language standard, but Perl doesn't currently assume anything in regard
       to this.)

       What this means is that unless the "/a" modifier is in effect "\d" not only matches the
       digits '0' - '9', but also Arabic, Devanagari, and digits from other languages.  This may
       cause some confusion, and some security issues.

       Some digits that "\d" matches look like some of the [0-9] ones, but have different values.
       For example, BENGALI DIGIT FOUR (U+09EA) looks very much like an ASCII DIGIT EIGHT
       (U+0038).  An application that is expecting only the ASCII digits might be misled, or if
       the match is "\d+", the matched string might contain a mixture of digits from different
       writing systems that look like they signify a number different than they actually do.
       "num()" in Unicode::UCD can be used to safely calculate the value, returning "undef" if
       the input string contains such a mixture.

       What "\p{Digit}" means (and hence "\d" except under the "/a" modifier) is
       "\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 "CIRCLED DIGIT ONE" or subscripts, or are from writing
       systems that lack all ten digits.

       The design intent is for "\d" to exactly match the set of characters that can safely be
       used with "normal" big-endian positional decimal syntax, where, for example 123 means one
       'hundred', plus two 'tens', plus three 'ones'.  This positional notation does not
       necessarily apply to characters that match the other type of "digit",
       "\p{Numeric_Type=Digit}", and so "\d" doesn't match them.

       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>.)

       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.

       If the "/a" modifier is in effect ...
           "\w" matches the 63 characters [a-zA-Z0-9_].

       otherwise ...
           For code points above 255 ...
               "\w" matches the same as "\p{Word}" matches in this range.  That is, it matches
               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.

           For code points below 256 ...
               if locale rules are in effect ...
                   "\w" matches the platform's native underscore character plus whatever the
                   locale considers to be alphanumeric.

               if Unicode rules are in effect or if on an EBCDIC platform ...
                   "\w" matches exactly what "\p{Word}" matches.

               otherwise ...
                   "\w" matches [a-zA-Z0-9_].

       Which rules apply are determined as described in "Which character set modifier is in
       effect?" in perlre.

       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.

       If the "/a" modifier is in effect ...
           "\s" matches the 5 characters [\t\n\f\r ]; that is, the horizontal tab, the newline,
           the form feed, the carriage return, and the space.  (Note that it doesn't match the
           vertical tab, "\cK" on ASCII platforms.)

       otherwise ...
           For code points above 255 ...
               "\s" matches exactly the code points above 255 shown with an "s" column in the
               table below.

           For code points below 256 ...
               if locale rules are in effect ...
                   "\s" matches whatever the locale considers to be whitespace.  Note that this
                   is likely to include the vertical space, unlike non-locale "\s" matching.

               if Unicode rules are in effect or if on an EBCDIC platform ...
                   "\s" matches exactly the characters shown with an "s" column in the table
                   below.

               otherwise ...
                   "\s" matches [\t\n\f\r ].  Note that this list doesn't include the non-
                   breaking space.

       Which rules apply are determined as described in "Which character set modifier is in
       effect?" in perlre.

       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" (and "\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.  For example, the
       vertical tab ("\x0b") is not matched by "\s", it is however considered vertical
       whitespace.

       The following table is a complete listing of characters matched by "\s", "\h" and "\v" as
       of Unicode 6.0.

       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 may or may not match "\s" depending on the rules in
           effect.  See the beginning of this section.

       \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".

       If neither the "/a" modifier nor locale rules are in effect, the use of a Unicode property
       will force the regular expression into using Unicode rules.

       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
       that are 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_Letter"s. They're actually "Letter_Number"s.)  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.

       It is worth emphasizing 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+".  But be aware of the security considerations in doing so, as mentioned above.

   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.

        -------

       * There is an exception to a bracketed character class matching only a single character.
       When the class is to match caselessely under "/i" matching rules, and a character inside
       the class matches a multiple-character sequence caselessly under Unicode rules, the class
       (when not inverted) will also match that sequence.  For example, Unicode says that the
       letter "LATIN SMALL LETTER SHARP S" should match the sequence "ss" under "/i" rules.
       Thus,

        'ss' =~ /\A\N{LATIN SMALL LETTER SHARP S}\z/i             # Matches
        'ss' =~ /\A[aeioust\N{LATIN SMALL LETTER SHARP S}]\z/i    # Matches

       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.  The class is said to be "negated" or
       "inverted".

       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.

       In inverted bracketed character classes, Perl ignores the Unicode rules that normally say
       that a given character matches a sequence of multiple characters under caseless "/i"
       matching, which otherwise could be highly confusing:

        "ss" =~ /^[^\xDF]+$/ui;

       This should match any sequences of characters that aren't "\xDF" nor what "\xDF" matches
       under "/i".  "s" isn't "\xDF", but Unicode says that "ss" is what "\xDF" matches under
       "/i".  So which one "wins"? Do you fail the match because the string has "ss" or accept it
       because it has an "s" followed by another "s"?

       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.

       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" counterparts always assume Unicode rules are in effect.  On ASCII platforms,
       this means they assume that the code points from 128 to 255 are Latin-1, and that means
       that using them under locale rules is unwise unless the locale is guaranteed to be Latin-1
       or UTF-8.  In contrast, the POSIX character classes are useful under locale rules.  They
       are affected by the actual rules in effect, as follows:

       If the "/a" modifier, is in effect ...
           Each of the POSIX classes matches exactly the same as their ASCII-range counterparts.

       otherwise ...
           For code points above 255 ...
               The POSIX class matches the same as its Full-range counterpart.

           For code points below 256 ...
               if locale rules are in effect ...
                   The POSIX class matches according to the locale.

               if Unicode rules are in effect or if on an EBCDIC platform ...
                   The POSIX class matches the same as the Full-range counterpart.

               otherwise ...
                   The POSIX class matches the same as the ASCII range counterpart.

       Which rules apply are determined as described in "Which character set modifier is in
       effect?" in perlre.

       It is proposed to change this behavior in a future release of Perl so that whether or not
       Unicode rules are in effect would not change the behavior:  Outside of locale or an EBCDIC
       code page, the POSIX classes would behave like 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{PosixXDigit}      \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 code points 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
           code pointss 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 if Unicode
           rules are in effect, "[[: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 in non-locale matching,
           "\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 of POSIX character classes

       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}    \W

       The backslash sequence can mean either ASCII- or Full-range Unicode, depending on various
       factors as described in "Which character set modifier is in effect?" in perlre.

       [= =] 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'.