The character ‘.’ matches any single character.
indicates that the regular expression should match one or more occurrences of the previous atom or regexp.
indicates that the regular expression should match zero or one occurrence of the previous atom or regexp.
matches a ‘+’
matches a ‘?’.
Bracket expressions are used to match ranges of characters. Bracket expressions where the range is backward, for example ‘[z-a]’, are ignored. Within square brackets, ‘\’ is taken literally. Character classes are not supported, so for example you would need to use ‘[0-9]’ instead of ‘[[:digit:]]’.
GNU extensions are supported:
Grouping is performed with backslashes followed by parentheses ‘\(’, ‘\)’. 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 character ‘^’ only represents the beginning of a string when it appears:
The character ‘$’ only represents the end of a string when it appears:
‘*’, ‘+’ 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.