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.

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Regular expression
Reference

Regex quick reference

^ · $
Start/end; m = lines.
[abc] · \d
Listed character / digit.
* · + · ?
Zero or more, one or more, optional.
(...)
Capture a numbered group.
(?<name>...)
Named capture.
(?=...)
Ahead; lookbehind varies.
i · m · s
Ignore case / line anchors / dot-newline.
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the RegExp of this browser (version not pinned)

Not available in JavaScript · This browser: x · ignore pattern whitespace; a · ASCII-only classes; U · lazy by default; J · duplicate names. u and v cannot be combined.

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Verified Fixture FX-G03

What it does

A regex that works in one place but not in your code usually meets a different dialect, different flags or an extra escaping layer.

How to use it

  1. Select the engine your code uses.
  2. Load the failing case: JavaScript named-group syntax in Python.
  3. Load the corrected pattern, then export code for your language.

Verified example

As written

Text "TX-00057"

Python re · Local WASM
(?<id>TX-\d{5})

Result: rejected: invalid pattern

Corrected

Text "TX-00057"

Python re · Local WASM
(?P<id>TX-\d{5})

Result: "TX-00057"

Recorded by running fixture FX-G03 on : rp.js@1 on Node v24.21.0 (V8), v24.21.0, CPython 3.14.2 (Pyodide 314.0.7), PCRE2 10.48, go1.27.1. JavaScript results come from the rp.js@1 core on Node’s V8; your browser runs the same core.

Engines

Limits