Source of: /manual/en/regexp.reference.squarebrackets.php
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<div id="regexp.reference.squarebrackets" class="section">
<h2 class="title">Square brackets</h2>
<p class="para">
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.
</p>
<p class="para">
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.
</p>
<p class="para">
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.
</p>
<p class="para">
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.
</p>
<p class="para">
The newline character is never treated in any special way in
character classes, whatever the setting of the <a href="reference.pcre.pattern.modifiers.php" class="link">PCRE_DOTALL</a>
or <a href="reference.pcre.pattern.modifiers.php" class="link">PCRE_MULTILINE</a>
options is. A class such as [^a] will always match a newline.
</p>
<p class="para">
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.
</p>
<p class="para">
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.
</p>
<p class="para">
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.
</p>
<p class="para">
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.
</p>
<p class="para">
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.
</p>
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