charnames.pm: Clarify comments

[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. The 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 Digits

C<\d> matches a single character that is considered to be a decimal I<digit>.
What is considered a decimal digit depends on several factors, detailed
below in L</Locale, EBCDIC, Unicode and UTF-8>.  If those factors
indicate a Unicode interpretation, C<\d> not only matches the digits
'0' - '9', but also Arabic, Devanagari and digits from other languages.
Otherwise, if there is a locale in effect, it will match whatever
characters the locale considers decimal digits.  Without a locale, C<\d>
matches just the digits '0' to '9'.

Unicode digits may cause some confusion, and some security issues.  In UTF-8
strings, C<\d> matches the same characters matched by
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 subscripts.

The design intent is for C<\d> to match all the 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 L<http://www.unicode.org/notes/tn21>.)

Some of the non-European digits that C<\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 L<Unicode::UCD/charscript()>.

Any character that isn't matched by C<\d> will be matched by C<\D>.

=head3 Word characters

A C<\w> matches a single alphanumeric character (an alphabetic character, or a
decimal digit) or an underscore (C<_>), not a whole word.  To match a whole
word, use C<\w+>.  This isn't the same thing as matching an English word, but 
is the same as a string of Perl-identifier characters.  What is considered a
word character depends on several factors, detailed below in L</Locale,
EBCDIC, Unicode and UTF-8>.  If those factors indicate a Unicode
interpretation, C<\w> matches the characters that are considered word
characters in the Unicode database. That is, it not only matches ASCII letters,
but also Thai letters, Greek letters, etc.  If a Unicode interpretation
is not indicated, C<\w> matches those characters that are considered
word characters by the current locale or EBCDIC code page.  Without a
locale or EBCDIC code page, C<\w> matches the ASCII letters, digits and
the underscore.

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 Unicode properties, "ID_Start", ID_Continue", "XID_Start", and
"XID_Continue".  See L<http://unicode.org/reports/tr31>.

Any character that isn't matched by C<\w> will be matched by C<\W>.

=head3 Whitespace

C<\s> matches any single character that is considered whitespace.  The exact
set of characters matched by C<\s> depends on several factors, detailed
below in L</Locale, EBCDIC, Unicode and UTF-8>.  If those factors
indicate a Unicode interpretation, C<\s> matches what is considered
whitespace in the Unicode database; the complete list is in the table
below. Otherwise, if there is a locale or EBCDIC code page in effect,
C<\s> matches whatever is considered whitespace by the current locale or
EBCDIC code page. Without a locale or EBCDIC code page, C<\s> matches
the horizontal tab (C<\t>), the newline (C<\n>), the form feed (C<\f>),
the carriage return (C<\r>), and the space.  (Note that it doesn't match
the vertical tab, C<\cK>.)  Perhaps the most notable possible surprise
is that C<\s> matches a non-breaking space only if a Unicode
interpretation is indicated, or the locale or EBCDIC code page that is
in effect has that character.

Any character that isn't matched by C<\s> will be matched by C<\S>.

C<\h> will match any character that is considered horizontal whitespace;
this includes the space and the tab characters and a number other characters,
all of which are listed in the table below.  C<\H> will match any character
that is not considered horizontal whitespace.

C<\v> will match any character that is considered vertical whitespace;
this includes the carriage return and line feed characters (newline) plus several
other characters, all listed in the table below.
C<\V> will match any character that is not considered vertical whitespace.

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).
Details are discussed in L<perlrebackslash>.

Note that unlike C<\s>, C<\d> and C<\w>, C<\h> and C<\v> always match
the same characters, without regard to other factors, such as if the
source string is in UTF-8 format or not.

One might think that C<\s> is equivalent to C<[\h\v]>. This is not true.  The
vertical tab (C<"\x0b">) is not matched by C<\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 C<"\x85">) and the no-break space (ASCII-platform
C<"\xA0">) characters are not matched by C<\s>, but are by C<\v> and C<\h>
respectively.  If the source string is in UTF-8 format, both the next line and
the no-break space are matched by C<\s>.

The following table is a complete listing of characters matched by
C<\s>, C<\h> and C<\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 C<\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

=over 4

=item [1]

NEXT LINE and NO-BREAK SPACE only match C<\s> if the source string is in
UTF-8 format, or the locale or EBCDIC code page that is in effect includes them.

=back

It is worth noting that C<\d>, C<\w>, etc, match single characters, not
complete numbers or words. To match a number (that consists of integers),
use C<\d+>; to match a word, use C<\w+>.

=head3 \N

C<\N> is new in 5.12, and is experimental.  It, like the dot, will match 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 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 the 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 as 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>.

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


=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.


=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 will be 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 a string of 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.

=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<wide 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.

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 the 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 the characters between the two are 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
necessary 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 will be. 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 will be
considered a character that is to be matched literally. You have to escape the
hyphen with a backslash if you want to have 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.

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 a character that is not a
lowercase ASCII letter.

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
to have the caret as one of the characters you want to match, you either
have to escape the caret, or 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.

=head3 Backslash Sequences

You can put any backslash sequence character class (with the exception of
C<\N>) inside a bracketed character class, and it will act just
as if you put all the characters matched by the backslash sequence inside the
character class. For instance, C<[a-f\d]> will match 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<wide 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, though they currently suffer from
portability issues (see below and L<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 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 alphanumerical 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, will only match 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}> will match not just the ASCII alphabetic characters, but any
character in the entire Unicode character set that is considered to be
alphabetic.

(Each of the counterparts has various synonyms as well.
L<perluniprops/Properties accessible through \p{} and \P{}> lists all the
synonyms, plus all the characters matched by each of the ASCII-range
properties.  For example C<\p{AHex}> is a synonym for C<\p{ASCII_Hex_Digit}>,
and any C<\p> property name can be prefixed with "Is" such as C<\p{IsAlpha}>.)

Both the C<\p> forms are unaffected by any locale that is 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.  If the source string is
in UTF-8 format, the POSIX classes (with the exception of C<[[:punct:]]>, see
Note [5] below) behave like their "Full-range" Unicode counterparts.  If 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 UTF8ness 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
disagree with this proposal, send email to C<perl5-porters@perl.org>.

 [[:...:]]      ASCII-range        Full-range  backslash  Note
                 Unicode            Unicode    sequence
 -----------------------------------------------------
   alpha      \p{PosixAlpha}       \p{Alpha}
   alnum      \p{PosixAlnum}       \p{Alnum}
   ascii      \p{ASCII}          
   blank      \p{PosixBlank}       \p{Blank} =             [1]
                                   \p{HorizSpace}  \h      [1]
   cntrl      \p{PosixCntrl}       \p{Cntrl}               [2]
   digit      \p{PosixDigit}       \p{Digit}       \d
   graph      \p{PosixGraph}       \p{Graph}               [3]
   lower      \p{PosixLower}       \p{Lower}
   print      \p{PosixPrint}       \p{Print}               [4]
   punct      \p{PosixPunct}       \p{Punct}               [5]
              \p{PerlSpace}        \p{SpacePerl}   \s      [6]
   space      \p{PosixSpace}       \p{Space}               [6]
   upper      \p{PosixUpper}       \p{Upper}
   word       \p{PerlWord}         \p{Word}        \w
   xdigit     \p{ASCII_Hex_Digit}  \p{XDigit}

=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 ordinals 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 ordinals from 128 through 159.

=item [3]

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

=item [4]

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

=item [5] (punct)

C<\p{PosixPunct}> and C<[[:punct:]]> in the ASCII range match all the
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:]]>).

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

When the matching string is in UTF-8 format, C<[[:punct:]]> matches what it
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
for a UTF-8 string, C<[[:punct:]]> matches all the characters that Unicode
considers to be punctuation, plus all the ASCII-range characters that Unicode
considers to be symbols.

=item [6]

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

=back

=head4 Negation
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{Digit}      \D
 [[:^space:]]   \P{PosixSpace}     \P{Space}
                \P{PerlSpace}      \P{SpacePerl}  \S
 [[:^word:]]    \P{PerlWord}       \P{Word}       \W

=head4 [= =] and [. .]

Perl will recognize the POSIX character classes C<[=class=]>, and
C<[.class.]>, but does not (yet?) support them.  Use of
such a construct will lead to an error.


=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'.  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'.


=head2 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, if the regular expression
is marked as having Unicode semantics, the locale that is in effect,
and if the program is running on an EBCDIC platform.

C<\w>, C<\d>, C<\s> and the POSIX character classes (and their negations,
including C<\W>, C<\D>, C<\S>) have this behaviour.  (Since the backslash
sequences C<\b> and C<\B> are defined in terms of C<\w> and C<\W>, they also are
affected.)

The rule is that if the source string is in UTF-8 format or the regular
expression is marked as indicating Unicode semantics (see the next
paragraph), the character classes match according to the Unicode
properties.  Otherwise, the character classes match according to
whatever locale or EBCDIC code page is in effect. If there is no locale
nor EBCDIC, they match the ASCII defaults (0 to 9 for C<\d>; 52 letters,
10 digits and underscore for C<\w>; etc.).

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 C<\N{U+...}> or C<\N{I<name>}>
construct.

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
L<perlunicode/The "Unicode Bug">.

For portability reasons, it may be better to not use C<\w>, C<\d>, C<\s>
or the POSIX character classes, and use the Unicode properties instead.

=head4 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.

=cut