The character ‘.’ matches any single character.
Bracket expressions are used to match ranges of characters. Bracket expressions where the range is backward, for example ‘[z-a]’, are invalid. Within square brackets, ‘\’ can be used to quote the following character. Character classes are supported; for example ‘[[:digit:]]’ will match a single decimal digit.
GNU extensions are supported:
Grouping is performed with parentheses ‘()’. An unmatched ‘)’ matches just itself. A backslash followed by a digit acts as a back-reference and matches the same thing as the previous grouped expression indicated by that number. For example ‘\2’ matches the second group expression. The order of group expressions is determined by the position of their opening parenthesis ‘(’.
The alternation operator is ‘|’.
The characters ‘^’ and ‘$’ always represent the beginning and end of a string respectively, except within square brackets. Within brackets, ‘^’ can be used to invert the membership of the character class being specified.
‘*’, ‘+’ and ‘?’ are special at any point in a regular expression except:
The longest possible match is returned; this applies to the regular expression as a whole and (subject to this constraint) to subexpressions within groups.