Negative lookbehind

A negative lookbehind assertion asserts true if the pattern inside the lookbehind is not matched.

Here is its syntax:


For example, (?<!xyz)abc asserts that there cannot be the string, xyz, just before matching the string, abc.

Here are a few important points about lookaround regex patterns:

  • Lookaround patterns are atomic. Like atomic groups, once a lookaround pattern is matched, the regex engine exits immediately from that lookaround, returning just a true or false assertion.
  • Lookaround patterns don't move from the current position. All patterns are evaluated from the current position. The position remains the same after the lookaround assertions are completed.
  • If a regular expression uses multiple lookaround ...

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