- True if grouped expression is (or is not) behind the current position.
- Similar to lookahead assertions.
- Group is not included in the match or captured.
- Javascript support is available.
- Metacharacters are
?<= Group is a positive lookbehind assertion.
?<! Group is a negative lookbehind assertion.
- A example:
/(?<=base)ball/ matches “ball” in “baseball”, but not “football”.
- Looks backwards at the word it initially matches.
/(?<!base)ball/ matches “ball” in “football”, but not “baseball”./(?<=\bfor\s)\b\w+/ matches first word after “for”./\b\w+(?<!er)\b/ matches words not ending in “er”.- An example is “I like baseball and football.”
/(?<=base)ball/ will match the ball in “baseball”, but not “football”.
- Changing the regex to
/(?<!base)ball/ then matches the “ball” in “football” instead.
- Next example is:
John Smith
Mr. Smith
Ms. Smith
Mrs. Smith
Mr. John Smith
- Can match “Mr”, “Ms” and “Mrs” with
/(?<=(Mr|Ms|Mrs)\. )Smith/
- Most regex engines do not allow expressions with variable widths.
- Must have the same width.
- Perl, Ruby and Java support alternations of different widths.