perlrecharclass.pod

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=head1 NAME

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, backslashed sequences, and the bracketed form.

=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 with the I<single line> modifier: either
for the entire regular expression using the C</s> modifier, or
locally using C<(?s)>.

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 Backslashed sequences

Perl regular expressions contain many backslashed sequences that
constitute a character class. That is, they will match a single
character, if that character belongs to a specific set of characters
(defined by the sequence). A backslashed sequence is a sequence of
characters starting with a backslash. Not all backslashed sequences
are character class; for a full list, see L<perlrebackslash>.

Here's a list of the backslashed sequences, which are discussed in
more detail below.

 \d             Match a digit character.
 \D             Match a non-digit character.
 \w             Match a "word" character.
 \W             Match a non-"word" character.
 \s             Match a white space character.
 \S             Match a non-white space character.
 \h             Match a horizontal white space character.
 \H             Match a character that isn't horizontal white space.
 \v             Match a vertical white space character.
 \V             Match a character that isn't vertical white space.
 \pP, \p{Prop}  Match a character matching a Unicode property.
 \PP, \P{Prop}  Match a character that doesn't match a Unicode property.

=head3 Digits

C<\d> matches a single character that is considered to be a I<digit>.
What is considered a digit depends on the internal encoding of
the source string. If the source string is in UTF-8 format, 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 digits. Without
a locale, C<\d> matches the digits '0' to '9'.
See L</Locale, Unicode and UTF-8>.

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

=head3 Word characters

C<\w> matches a single I<word> character: an alphanumeric character
(that is, an alphabetic character, or a digit), or the underscore (C<_>).
What is considered a word character depends on the internal encoding
of the string. If it's in UTF-8 format, C<\w> matches those 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 the source string isn't in UTF-8 format, C<\w> matches those characters
that are considered word characters by the current locale. Without
a locale in effect, C<\w> matches the ASCII letters, digits and the
underscore.

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

=head3 White space

C<\s> matches any single character that is consider white space. In the
ASCII range, C<\s> matches the horizontal tab (C<\t>), the new line
(C<\n>), the form feed (C<\f>), the carriage return (C<\r>), and the
space (the vertical tab, C<\cK> is not matched by C<\s>).  The exact set
of characters matched by C<\s> depends on whether the source string is
in UTF-8 format. If it is, C<\s> matches what is considered white space
in the Unicode database. Otherwise, if there is a locale in effect, C<\s>
matches whatever is considered white space by the current locale. Without
a locale, C<\s> matches the five characters mentioned in the beginning
of this paragraph.  Perhaps the most notable difference is that C<\s>
matches a non-breaking space only if the non-breaking space is in a
UTF-8 encoded string.

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 white space;
this includes the space and the tab characters. C<\H> will match any character
that is not considered horizontal white space.

C<\v> will match any character that is considered vertical white space;
this includes the carriage return and line feed characters (newline).
C<\V> will match any character that is not considered vertical white space.

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. Details are discussed in L<perlrebackslash>.

C<\h>, C<\H>, C<\v>, C<\V>, and C<\R> are new in perl 5.10.0.

Note that unlike C<\s>, C<\d> and C<\w>, C<\h> and C<\v> always match
the same characters, regardless whether the source string is in UTF-8
format or not. The set of characters they match is also not influenced
by locale.

One might think that C<\s> is equivalent with C<[\h\v]>. This is not true.
The vertical tab (C<"\x0b">) is not matched by C<\s>, it is however
considered vertical white space. Furthermore, if the source string is
not in UTF-8 format, the next line (C<"\x85">) and the no-break space
(C<"\xA0">) 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>.

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.

 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.

=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 Unicode Properties

C<\pP> and C<\p{Prop}> are character classes to match characters that
fit given Unicode classes. One letter classes can be used in the C<\pP>
form, with the class name following the C<\p>, otherwise, the property
name is enclosed in braces, and follows the C<\p>. For instance, a
match for a number can be written as C</\pN/> or as C</\p{Number}/>.
Lowercase letters are matched by the property I<LowercaseLetter> which
has as short form I<Ll>. They have to be written as C</\p{Ll}/> or
C</\p{LowercaseLetter}/>. 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 a list of possible properties, see
L<perlunicode/Unicode Character Properties>. It is also possible to
defined 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 white space.
 "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-white space.

 " "  =~  /\h/      # Match, space is horizontal white space.
 " "  =~  /\v/      # No match, space is not vertical white space.
 "\r" =~  /\v/      # Match, a return is vertical white space.

 "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 Laoian character.


=head2 Bracketed Character Classes

The third form of character class you can use in Perl regular expressions
is the bracketed form. In its simplest form, it lists the characters
that may be matched inside square brackets, like this: C<[aeiou]>.
This matches one of C<a>, C<e>, C<i>, C<o> or C<u>. Just as the other
character classes, exactly one character will be matched. To match
a longer string consisting of characters mentioned in the characters
class, follow the character class with a quantifier. For instance,
C<[aeiou]+> matches a string of one or more lowercase ASCII 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<*> 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