Character classes

An opening square bracket introduces a character
class, terminated by a closing square bracket. A closing square
bracket on its own is not special. If a closing square bracket is
required as a member of the class, it should be the first data
character in the class (after an initial circumflex, if present) or
escaped with a backslash.

A character class matches a single character in the
subject; the character must be in the set of characters defined by
the class, unless the first character in the class is a circumflex,
in which case the subject character must not be in the set defined
by the class. If a circumflex is actually required as a member of
the class, ensure it is not the first character, or escape it with
a backslash.

For example, the character class [aeiou] matches
any lower case vowel, while [^aeiou] matches any character that is
not a lower case vowel. Note that a circumflex is just a convenient
notation for specifying the characters which are in the class by
enumerating those that are not. It is not an assertion: it still
consumes a character from the subject string, and fails if the
current pointer is at the end of the string.

When case-insensitive (caseless) matching is set,
any letters in a class represent both their upper case and lower
case versions, so for example, an insensitive [aeiou] matches “A”
as well as “a”, and an insensitive [^aeiou] does not match “A”,
whereas a sensitive (caseful) version would.

The newline character is never treated in any
special way in character classes, whatever the setting of the
PCRE_DOTALL or PCRE_MULTILINE options is. A class such as [^a] will
always match a newline.

The minus (hyphen) character can be used to specify
a range of characters in a character class. For example, [d-m]
matches any letter between d and m, inclusive. If a minus character
is required in a class, it must be escaped with a backslash or
appear in a position where it cannot be interpreted as indicating a
range, typically as the first or last character in the class.

It is not possible to have the literal character
“]” as the end character of a range. A pattern such as [W-]46] is
interpreted as a class of two characters (“W” and “-“) followed by
a literal string “46]”, so it would match “W46]” or “-46]”.
However, if the “]” is escaped with a backslash it is interpreted
as the end of range, so [W-\]46] is interpreted as a single class
containing a range followed by two separate characters. The octal
or hexadecimal representation of “]” can also be used to end a
range.

Ranges operate in ASCII collating sequence. They
can also be used for characters specified numerically, for example
[\000-\037]. If a range that includes letters is used when
case-insensitive (caseless) matching is set, it matches the letters
in either case. For example, [W-c] is equivalent to
[][\^_`wxyzabc], matched case-insensitively, and if character
tables for the “fr” locale are in use, [\xc8-\xcb] matches accented
E characters in both cases.

The character types \d, \D, \s, \S, \w, and \W may
also appear in a character class, and add the characters that they
match to the class. For example, [\dABCDEF] matches any hexadecimal
digit. A circumflex can conveniently be used with the upper case
character types to specify a more restricted set of characters than
the matching lower case type. For example, the class [^\W_] matches
any letter or digit, but not underscore.

All non-alphanumeric characters other than \, -, ^
(at the start) and the terminating ] are non-special in character
classes, but it does no harm if they are escaped. The pattern
terminator is always special and must be escaped when used within
an expression.

Perl supports the POSIX notation for character
classes. This uses names enclosed by [: and :]
within the enclosing square brackets. PCRE also supports this
notation. For example, [01[:alpha:]%] matches “0”, “1”,
any alphabetic character, or “%”. The supported class names
are:

The space characters are HT (9), LF (10), VT (11), FF
(12), CR (13), and space (32). Notice that this list includes the
VT character (code 11). This makes “space” different to
\s, which does not include VT (for Perl compatibility).

The name word is a Perl extension, and
blank is a GNU extension from Perl 5.8. Another Perl
extension is negation, which is indicated by a ^ character
after the colon. For example, [12[:^digit:]] matches “1”,
“2”, or any non-digit.

In UTF-8 mode, characters with values greater than
128 do not match any of the POSIX character classes. As of PHP
5.3.0 and libpcre 8.10 some character classes are changed to use
Unicode character properties, in which case the mentioned
restriction does not apply. Refer to the » PCRE(3) manual for details.

Unicode character properties can appear inside a
character class. They can not be part of a range. The minus
(hyphen) character after a Unicode character class will match
literally. Trying to end a range with a Unicode character property
will result in a warning.