Why Your Regex Does Not Match

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

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Workspace actions
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.
Browse the full reference

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No regex yet.

⋯

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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Test text

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Verified Fixture FX-G01

What it does

When a regex finds nothing, the usual causes are anchors that demand the whole text, missing flags, or another engine’s syntax.

How to use it

  1. Load the failing case and read the empty result.
  2. Check anchors, flags and the engine.
  3. Load the corrected pattern and compare.

Verified example

As written

Text "cat dog"

JavaScript · This browser
^cat$

Result: no match

Corrected

Text "cat dog"

JavaScript · This browser
\bcat\b

Result: "cat"

Recorded by running fixture FX-G01 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