MEPX
Chapter 7 of 10All chapters
  1. 1What a regex is
  2. 2Literals and classes
  3. 3Quantifiers
  4. 4Anchors and boundaries
  5. 5Groups and alternation
  6. 6Escaping
  7. 7Lookaround
  8. 8Flags
  9. 9When regex goes wrong
  10. 10Writing readable patterns

Chapter 7 of 10

Lookaround

Checking without consuming.

Four forms

A lookahead checks that something does or does not follow, and a lookbehind checks what precedes, without including it in the match.

  • (?=...) positive ahead, (?!...) negative ahead.
  • (?<=...) positive behind, (?<!...) negative behind.

When it helps

Password rules and matching a word only when it is not followed by something are the usual cases. Lookbehind support varies between flavours, so check before relying on it.