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

The sequences \a , \c , \e , \f , \n , \N{NAME}, \N{U+hex char}, \r , \t , and \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 [ is not special inside a character class, unless it's the start of a POSIX character class (see POSIX Character Classes below). It normally does not need escaping.

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

However, if the ] is the 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 ].