Lookaround

Code

(?=abc)     positive lookahead
(?!abc)     negative lookahead
(?<=abc)    positive lookbehind
(?<!abc)    negative lookbehind
a(?=b)      a followed by b
a(?!b)      a not followed by b
(?<=a)b     b preceded by a
(?<!a)b     b not preceded by a

Explanation

Lookaround assertions match a position based on what follows (lookahead) or precedes (lookbehind) without consuming characters. Positive forms require the pattern to match; negative forms require it not to match. Modern JavaScript supports variable-length lookbehind, but some engines (Java, older browsers) restrict lookbehind to fixed length.

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