Lookarounds are zero width assertions. The check for a regex (towards right or left of the current position - based on ahead or behind), succeeds or fails when a match is found (based on if it is positive or negative) and discards the matched portion. They don't consume any character - the matching for regex following them (if any), will start at the same cursor position.
Read regular-expression.info for more details.
Syntax:
(?=REGEX_1)REGEX_2
Match only if REGEX_1 matches; after matching REGEX_1, the match is discarded and searching for REGEX_2 starts at the same position.
example:
(?=[a-z0-9]{4}$)[a-z]{1,2}[0-9]{2,3}
REGEX_1 is [a-z0-9]{4}$
which matches four alphanumeric chars followed by end of line.
REGEX_2 is [a-z]{2}[0-9]{2}
which matches one or two letters followed by two or three digits.
REGEX_1 makes sure that the length of string is indeed 4, but doesn't consume any characters so that search for REGEX_2 starts at the same location. Now REGEX_2 makes sure that the string matches some other rules. Without look-ahead it would match strings of length three or five.
Syntax:
(?!REGEX_1)REGEX_2
Match only if REGEX_1 does not match; after checking REGEX_1, the search for REGEX_2 starts at the same position.
example:
(?!.*\bFWORD\b)\w{10,30}$
The look-ahead part checks for the FWORD
in the string and fails if it finds it. If it doesn't find FWORD
, the look-ahead succeeds and the following part verifies that the string's length is between 10 and 30 and that it contains only word characters a-zA-Z0-9_
Look-behind is similar to look-ahead: it just looks behind the current cursor position. Some regex flavors like javascript doesn't support look-behind assertions. And most flavors that support it (PHP, Python etc) require that look-behind portion be fixed length.
- Atomic groups basically discards/forgets the subsequent tokens in the group once a token matches. Check this page for examples of atomic groups