Regex Troubleshooting

Verified 2026-09-25 Each verified example was run in JavaScript, unless its card names other engines.

• Why Your Regex Does Not Match

  Find out why a regex does not match: anchors, flags and engine differences, with a runnable case.

  \bcat\b
  Open in workbench — Why Your Regex Does Not Match

• Why Your Regex Matches Too Much

  Find out why a regex matches too much, such as greedy .*, with a runnable case.

  <[^>]*>
  Open in workbench — Why Your Regex Matches Too Much

• Regex Works in a Tester but Not in Code

  Find out why a regex works in a tester but not in your code: dialect and escaping differences.

  (?P<id>TX-\d{5})
  Python re · Local WASM
  Open in workbench — Regex Works in a Tester but Not in Code

• Understand Zero-length Regex Matches

  Understand zero-length matches and why engines list them differently.

  a*
  JavaScript · This browserPython re · Local WASMPCRE2-8 · Local WASMGo regexp · Local WASM

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