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A positive lookahead (?=123) asserts the text is followed by the given pattern, without including the pattern in the match. Similarly, a positive lookbehind (?<=123) asserts the text is preceded by the given pattern. Replacing the = with ! negates the assertion. Input: 123456 123(?=456) mat...

A lookbehind can be used at the end of a pattern to ensure it ends or not in a certain way. ([a-z ]+|[A-Z ]+)(?<! ) matches sequences of only lowercase or only uppercase words while excluding trailing whitespace.

Some regex flavors (Perl, PCRE, Oniguruma, Boost) only support fixed-length lookbehinds, but offer the \K feature, which can be used to simulate variable-length lookbehind at the start of a pattern. Upon encountering a \K, the matched text up to this point is discarded, and only the text matching th...