add 5.12.5 to perlhist

[perl5.git] / pod / perlrecharclass.pod

=head1 NAME
X<character class>

perlrecharclass - Perl Regular Expression Character Classes

=head1 DESCRIPTION

The top level documentation about Perl regular expressions
is found in L<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.

=head2 The dot

The dot (or period), C<.> is probably the most used, and certainly
the most well-known character class. By default, a dot matches any
character, except for the newline. That default can be changed to
add matching the newline by using the I<single line> modifier: either
for the entire regular expression with the C</s> modifier, or
locally with C<(?s)>.  (The experimental C<\N> backslash sequence, described
below, matches any character except newline without regard to the
I<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)

=head2 Backslash sequences
X<\w> X<\W> X<\s> X<\S> X<\d> X<\D> X<\p> X<\P>
X<\N> X<\v> X<\V> X<\h> X<\H>
X<word> X<whitespace>

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 L<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

=head3 \N

C<\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 C<\N> is not influenced
by the I<single line> regular expression modifier (see L</The dot> above).  Note
that the form C<\N{...}> may mean something completely different.  When the
C<{...}> is a L<quantifier|perlre/Quantifiers>, it means to match a non-newline
character that many times.  For example, C<\N{3}> means to match 3
non-newlines; C<\N{5,}> means to match 5 or more non-newlines.  But if C<{...}>
is not a legal quantifier, it is presumed to be a named character.  See
L<charnames> for those.  For example, none of C<\N{COLON}>, C<\N{4F}>, and
C<\N{F4}> contain legal quantifiers, so Perl will try to find characters whose
names are respectively C<COLON>, C<4F>, and C<F4>.

=head3 Digits

C<\d> matches a single character considered to be a decimal I<digit>.
If the C</a> regular expression modifier is in effect, it matches [0-9].
Otherwise, it
matches anything that is matched by C<\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 C<\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 C</a> modifier is in effect C<\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 C<\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
C<\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.  L<Unicode::UCD/num()> can
be used to safely
calculate the value, returning C<undef> if the input string contains
such a mixture.

What C<\p{Digit}> means (and hence C<\d> except under the C</a>
modifier) is C<\p{General_Category=Decimal_Number}>, or synonymously,
C<\p{General_Category=Digit}>.  Starting with Unicode version 4.1, this
is the same set of characters matched by C<\p{Numeric_Type=Decimal}>.
But Unicode also has a different property with a similar name,
C<\p{Numeric_Type=Digit}>, which matches a completely different set of
characters.  These characters are things such as C<CIRCLED DIGIT ONE>
or subscripts, or are from writing systems that lack all ten digits.

The design intent is for C<\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",
C<\p{Numeric_Type=Digit}>, and so C<\d> doesn't match them.

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 L<http://www.unicode.org/notes/tn21>.)

Any character not matched by C<\d> is matched by C<\D>.

=head3 Word characters

A C<\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 C<\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.

=over

=item If the C</a> modifier is in effect ...

C<\w> matches the 63 characters [a-zA-Z0-9_].

=item otherwise ...

=over

=item For code points above 255 ...

C<\w> matches the same as C<\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 C<COMBINING TILDE> and the modifier letters, which
are generally used to add auxiliary markings to letters.

=item For code points below 256 ...

=over

=item if locale rules are in effect ...

C<\w> matches the platform's native underscore character plus whatever
the locale considers to be alphanumeric.

=item if Unicode rules are in effect or if on an EBCDIC platform ...

C<\w> matches exactly what C<\p{Word}> matches.

=item otherwise ...

C<\w> matches [a-zA-Z0-9_].

=back

=back

=back

Which rules apply are determined as described in L<perlre/Which character set modifier is in effect?>.

There are a number of security issues with the full Unicode list of word
characters.  See L<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 L</Unicode Properties>, C<\p{ID_Start}>,
C<\p{ID_Continue}>, C<\p{XID_Start}>, and C<\p{XID_Continue}>.  See
L<http://unicode.org/reports/tr31>.

Any character not matched by C<\w> is matched by C<\W>.

=head3 Whitespace

C<\s> matches any single character considered whitespace.

=over

=item If the C</a> modifier is in effect ...

C<\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, C<\cK> on ASCII platforms.)

=item otherwise ...

=over

=item For code points above 255 ...

C<\s> matches exactly the code points above 255 shown with an "s" column
in the table below.

=item For code points below 256 ...

=over

=item if locale rules are in effect ...

C<\s> matches whatever the locale considers to be whitespace.  Note that
this is likely to include the vertical space, unlike non-locale C<\s>
matching.

=item if Unicode rules are in effect or if on an EBCDIC platform ...

C<\s> matches exactly the characters shown with an "s" column in the
table below.

=item otherwise ...

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

=back

=back

=back

Which rules apply are determined as described in L<perlre/Which character set modifier is in effect?>.

Any character not matched by C<\s> is matched by C<\S>.

C<\h> matches any character considered horizontal whitespace;
this includes the platform's space and tab characters and several others
listed in the table below.  C<\H> matches any character
not considered horizontal whitespace.  They use the platform's native
character set, and do not consider any locale that may otherwise be in
use.

C<\v> matches any character considered vertical whitespace;
this includes the platform's carriage return and line feed characters (newline)
plus several other characters, all listed in the table below.
C<\V> matches any character not considered vertical whitespace.
They use the platform's native character set, and do not consider any
locale that may otherwise be in use.

C<\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 C<\v> instead (vertical whitespace).  It uses the platform's
native character set, and does not consider any locale that may
otherwise be in use.
Details are discussed in L<perlrebackslash>.

Note that unlike C<\s> (and C<\d> and C<\w>), C<\h> and C<\v> always match
the same characters, without regard to other factors, such as the active
locale or whether the source string is in UTF-8 format.

One might think that C<\s> is equivalent to C<[\h\v]>. This is not true.
The difference is that the vertical tab (C<"\x0b">) is not matched by
C<\s>; it is however considered vertical whitespace.

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

The first column gives the Unicode 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 C<\s> matching).

 0x0009        CHARACTER TABULATION   h s
 0x000a              LINE FEED (LF)    vs
 0x000b             LINE TABULATION    v
 0x000c              FORM FEED (FF)    vs
 0x000d        CARRIAGE RETURN (CR)    vs
 0x0020                       SPACE   h s
 0x0085             NEXT LINE (NEL)    vs  [1]
 0x00a0              NO-BREAK SPACE   h s  [1]
 0x1680            OGHAM SPACE MARK   h s
 0x180e   MONGOLIAN VOWEL SEPARATOR   h s
 0x2000                     EN QUAD   h s
 0x2001                     EM QUAD   h s
 0x2002                    EN SPACE   h s
 0x2003                    EM SPACE   h s
 0x2004          THREE-PER-EM SPACE   h s
 0x2005           FOUR-PER-EM SPACE   h s
 0x2006            SIX-PER-EM SPACE   h s
 0x2007                FIGURE SPACE   h s
 0x2008           PUNCTUATION SPACE   h s
 0x2009                  THIN SPACE   h s
 0x200a                  HAIR SPACE   h s
 0x2028              LINE SEPARATOR    vs
 0x2029         PARAGRAPH SEPARATOR    vs
 0x202f       NARROW NO-BREAK SPACE   h s
 0x205f   MEDIUM MATHEMATICAL SPACE   h s
 0x3000           IDEOGRAPHIC SPACE   h s

=over 4

=item [1]

NEXT LINE and NO-BREAK SPACE may or may not match C<\s> depending
on the rules in effect.  See
L<the beginning of this section|/Whitespace>.

=back

=head3 Unicode Properties

C<\pP> and C<\p{Prop}> are character classes to match characters that fit given
Unicode properties.  One letter property names can be used in the C<\pP> form,
with the property name following the C<\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 C<\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 C</\pN/> or as
C</\p{Number}/>, or as C</\p{Number=True}/>.
Lowercase letters are matched by the property I<Lowercase_Letter> which
has the short form I<Ll>. They need the braces, so are written as C</\p{Ll}/> or
C</\p{Lowercase_Letter}/>, or C</\p{General_Category=Lowercase_Letter}/>
(the underscores are optional).
C</\pLl/> is valid, but means something different.
It matches a two character string: a letter (Unicode property C<\pL>),
followed by a lowercase C<l>.

If locale rules are not in effect, the use of
a Unicode property will force the regular expression into using Unicode
rules, if it isn't already.

Note that almost all properties are immune to case-insensitive matching.
That is, adding a C</i> regular expression modifier does not change what
they match.  There are two sets that are affected.  The first set is
C<Uppercase_Letter>,
C<Lowercase_Letter>,
and C<Titlecase_Letter>,
all of which match C<Cased_Letter> under C</i> matching.
The second set is
C<Uppercase>,
C<Lowercase>,
and C<Titlecase>,
all of which match C<Cased> under C</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 C<Cased>, but
aren't considered to be letters, so they aren't C<Cased_Letter>s. They're
actually C<Letter_Number>s.)
This set also includes its subsets C<PosixUpper> and C<PosixLower>, both
of which under C</i> match C<PosixAlpha>.

For more details on Unicode properties, see L<perlunicode/Unicode
Character Properties>; for a
complete list of possible properties, see
L<perluniprops/Properties accessible through \p{} and \P{}>,
which notes all forms that have C</i> differences.
It is also possible to define your own properties. This is discussed in
L<perlunicode/User-Defined Character Properties>.

Unicode properties are defined (surprise!) only on Unicode code points.
A warning is raised and all matches fail on non-Unicode code points
(those above the legal Unicode maximum of 0x10FFFF).  This can be
somewhat surprising,

 chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Fails.
 chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Also fails!

Even though these two matches might be thought of as complements, they
are so only on Unicode code points.

=head4 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 C<\d>, C<\w>, etc, match single characters, not
complete numbers or words. To match a number (that consists of digits),
use C<\d+>; to match a word, use C<\w+>.  But be aware of the security
considerations in doing so, as mentioned above.

=head2 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: C<[aeiou]>.
This matches one of C<a>, C<e>, C<i>, C<o> or C<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 L<quantifier|perlre/Quantifiers>.  For
instance, C<[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 a
single character only.  When the class is to match caselessly under C</i>
matching rules, and a character that is explicitly mentioned inside the
class matches a
multiple-character sequence caselessly under Unicode rules, the class
(when not L<inverted|/Negation>) will also match that sequence.  For
example, Unicode says that the letter C<LATIN SMALL LETTER SHARP S>
should match the sequence C<ss> under C</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

For this to happen, the character must be explicitly specified, and not
be part of a multi-character range (not even as one of its endpoints).
(L</Character Ranges> will be explained shortly.)  Therefore,

 'ss' =~ /\A[\0-\x{ff}]\z/i        # Doesn't match
 'ss' =~ /\A[\0-\N{LATIN SMALL LETTER SHARP S}]\z/i    # No match
 'ss' =~ /\A[\xDF-\xDF]\z/i    # Matches on ASCII platforms, since \XDF
                               # is LATIN SMALL LETTER SHARP S, and the
                               # range is just a single element

Note that it isn't a good idea to specify these types of ranges anyway.

=head3 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 C<.>, C<*>, or C<(>) lose
their special meaning and can be used inside a character class without
the need to escape them. For instance, C<[()]> 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:
C<\>, C<^>, C<->, C<[> and C<]>, 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 C<\b> is special inside a bracketed character class. While
outside the character class, C<\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, C<\b> matches a
backspace character.

The sequences
C<\a>,
C<\c>,
C<\e>,
C<\f>,
C<\n>,
C<\N{I<NAME>}>,
C<\N{U+I<hex char>}>,
C<\r>,
C<\t>,
and
C<\x>
are also special and have the same meanings as they do outside a
bracketed character class.  (However, inside a bracketed character
class, if C<\N{I<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 C<[> is not special inside a character class, unless it's the start of a
POSIX character class (see L</POSIX Character Classes> below). It normally does
not need escaping.

A C<]> is normally either the end of a POSIX character class (see
L</POSIX Character Classes> below), or it signals the end of the bracketed
character class.  If you want to include a C<]> in the set of characters, you
must generally escape it.

However, if the C<]> is the I<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 ].

=head3 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 (C<->).
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, C<[0-9]> matches any ASCII digit, and C<[a-m]>
matches any lowercase letter from the first half of the 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.  C<['-?]> 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).


=head3 Negation

It is also possible to instead list the characters you do not want to
match. You can do so by using a caret (C<^>) as the first character in the
character class. For instance, C<[^a-z]> matches any character that is not a
lowercase ASCII letter, which therefore includes more than a million
Unicode code points.  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 don't list it first.

In inverted bracketed character classes, Perl ignores the Unicode rules
that normally say that certain characters should match a sequence of
multiple characters under caseless C</i> matching.  Following those
rules could lead to highly confusing situations:

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

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

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.

=head3 Backslash Sequences

You can put any backslash sequence character class (with the exception of
C<\N> and C<\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, C<[a-f\d]> matches any decimal digit, or any
of the lowercase letters between 'a' and 'f' inclusive.

C<\N> within a bracketed character class must be of the forms C<\N{I<name>}>
or C<\N{U+I<hex char>}>, and NOT be the form that matches non-newlines,
for the same reason that a dot C<.> 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!

=head3 POSIX Character Classes
X<character class> X<\p> X<\p{}>
X<alpha> X<alnum> X<ascii> X<blank> X<cntrl> X<digit> X<graph>
X<lower> X<print> X<punct> X<space> X<upper> X<word> X<xdigit>

POSIX character classes have the form C<[:class:]>, where I<class> is
name, and the C<[:> and C<:]> delimiters. POSIX character classes only appear
I<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 C<a>, C<l>, C<p> and C<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 C<\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,
C<\p{Alpha}> matches not just the ASCII alphabetic characters, but any
character in the entire Unicode character set considered alphabetic.
An entry in the column labelled "backslash sequence" is a (short)
equivalent.

 [[:...:]]      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}

=over 4

=item [1]

C<\p{Blank}> and C<\p{HorizSpace}> are synonyms.

=item [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 (C<DEL>) are control characters.

On EBCDIC platforms, it is likely that the code page will define C<[[:cntrl:]]>
to be the EBCDIC equivalents of the ASCII controls, plus the controls
that in Unicode have code points from 128 through 159.

=item [3]

Any character that is I<graphical>, that is, visible. This class consists
of all alphanumeric characters and all punctuation characters.

=item [4]

All printable characters, which is the set of all graphical characters
plus those whitespace characters which are not also controls.

=item [5]

C<\p{PosixPunct}> and C<[[:punct:]]> in the ASCII range match all
non-controls, non-alphanumeric, non-space characters:
C<[-!"#$%&'()*+,./:;<=E<gt>?@[\\\]^_`{|}~]> (although if a locale is in effect,
it could alter the behavior of C<[[:punct:]]>).

The similarly named property, C<\p{Punct}>, matches a somewhat different
set in the ASCII range, namely
C<[-!"#%&'()*,./:;?@[\\\]_{}]>.  That is, it is missing the nine
characters C<[$+E<lt>=E<gt>^`|~]>.
This is because Unicode splits what POSIX considers to be punctuation into two
categories, Punctuation and Symbols.

C<\p{XPosixPunct}> and (under Unicode rules) C<[[:punct:]]>, match what
C<\p{PosixPunct}> matches in the ASCII range, plus what C<\p{Punct}>
matches.  This is different than strictly matching according to
C<\p{Punct}>.  Another way to say it is that
if Unicode rules are in effect, C<[[:punct:]]> matches all characters
that Unicode considers punctuation, plus all ASCII-range characters that
Unicode considers symbols.

=item [6]

C<\p{SpacePerl}> and C<\p{Space}> differ only in that in non-locale
matching, C<\p{Space}> additionally
matches the vertical tab, C<\cK>.   Same for the two ASCII-only range forms.

=back

There are various other synonyms that can be used besides the names
listed in the table.  For example, C<\p{PosixAlpha}> can be written as
C<\p{Alpha}>.  All are listed in
L<perluniprops/Properties accessible through \p{} and \P{}>,
plus all characters matched by each ASCII-range property.

Both the C<\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:

=over

=item If the C</a> modifier, is in effect ...

Each of the POSIX classes matches exactly the same as their ASCII-range
counterparts.

=item otherwise ...

=over

=item For code points above 255 ...

The POSIX class matches the same as its Full-range counterpart.

=item For code points below 256 ...

=over

=item if locale rules are in effect ...

The POSIX class matches according to the locale, except that
C<word> uses the platform's native underscore character, no matter what
the locale is.

=item if Unicode rules are in effect or if on an EBCDIC platform ...

The POSIX class matches the same as the Full-range counterpart.

=item otherwise ...

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

=back

=back

=back

Which rules apply are determined as described in
L<perlre/Which character set modifier is in effect?>.

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 C<perl5-porters@perl.org>.

=head4 Negation of POSIX character classes
X<character class, 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 (C<^>).
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 L<perlre/Which character set modifier is in effect?>.

=head4 [= =] and [. .]

Perl recognizes the POSIX character classes C<[=class=]> and
C<[.class.]>, but does not (yet?) support them.  Any attempt to use
either construct raises an exception.

=head4 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' and '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 OR adds the digits, leaving only the
                          # letters 'a' to 'f' and 'A' to 'F' excluded.