Regular Expressions Lookahead and Lookbehind
Syntax
- Positive lookahead:
(?=pattern) - Negative lookahead:
(?!pattern) - Positive lookbehind:
(?<=pattern) - Negative lookbehind:
(?<!pattern)
Remarks
Not supported by all regex engines.
Additionally, many regex engines limit the patterns inside lookbehinds to fixed-length strings. For example the pattern (?<=a+)b should match the b in aaab but throws an error in Python.
Capturing groups are allowed and work as expected, including backreferences. The lookahead/lookbehind itself is not a capturing group, however.
