How do I test a regex pattern online?

Type the pattern (without / slashes), add flags like g or i if needed, paste your test string, and run the tool above. You get the match count and every matched substring instantly — free, no signup.

Is there a free regex checker online?

Yes — this tool is free with no signup. It validates your pattern with the regulex engine (precise error positions, not just "invalid") and tests it against your text using real JavaScript regex semantics.

How do I validate a regular expression before using it in code?

Paste the pattern into the tester and run it without a test string focus: any syntax problem is reported with its exact position and a plain-English explanation, so you can fix it before it ships.

Does this regex tester use JavaScript regex syntax?

Yes — validation targets the JavaScript regex grammar and matching runs on your JS engine's native RegExp, so the results match what the pattern does in actual JavaScript code, including flags like g, i, m, s, u and y.

What is a regular expression tester?

A regular expression tester lets you try a regex pattern against sample text and see what it matches before using it in code. This one adds syntax validation with exact error positions, a match count, and the list of matched substrings.

Why does my regex show as invalid here?

The regulex engine is deliberately strict in documented ways: octal escapes are rejected and \N is always treated as a backreference (so \1 with no group 1 is an error). Fix the reported position, and note the "v" (unicodeSets) flag is not supported — use "u" instead.

Regex Tester: JavaScript RegExp Checker Online

What is Regex tester?

Regex Tester — Test JavaScript regex patterns against sample text with this free regex tester. Regulex-powered syntax errors plus exact match counts — all client-side.

Syntax is validated by regulex (vendored open-source JavaScript regex parser, MIT), which reports precise typed errors with the exact position — e.g. ParenMismatch for an unclosed group. Matching itself runs on your JavaScript engine's native RegExp, so results are identical to what the pattern does in real JS code. The test string is capped at 50,000 characters; the matches list shows the first 500 matches while the count stays exact. Everything runs in your browser — nothing is uploaded.

Why use this regex tester?

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Built for regex tester

Test JavaScript regex patterns against sample text with this free regex tester. Regulex-powered syntax errors plus exact match counts — all client-side.

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Transparent & honest

Validation follows the regulex engine, which is stricter than some browsers by design: no octal escapes, and decimal escapes are always treated as backreferences.

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Private by design

Everything runs in your browser. Your inputs never leave your device.

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Works everywhere

Fully responsive — perfect on desktop, tablet, and mobile. No app needed.

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Regularly updated

Regularly maintained and reviewed to ensure accuracy.

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Free forever

No accounts, no limits, no watermarks. Use Regex Tester as much as you need.

How to use Regex Tester

1Type your pattern into the "Regular expression" box — without the surrounding / slashes.
2Add flags like g, i or m in the "Flags" box if you need them; leave it empty for a single case-sensitive match.
3Paste the text you want to test into the "Test string" box.
4Run the tool: the regulex engine validates the syntax first — fix any reported error position, then read the match count and matched substrings.
5Tune the pattern and re-run until the matches list shows exactly what you want to capture.

What can you do with Regex Tester?

Test regex pattern online — Test JavaScript regex patterns against sample text with this free regex tester.
Free regex checker online — Test JavaScript regex patterns against sample text with this free regex tester.
Validate regular expression before using in code — Test JavaScript regex patterns against sample text with this free regex tester.
Javascript regex syntax — Test JavaScript regex patterns against sample text with this free regex tester.

Good to know

Validation follows the regulex engine, which is stricter than some browsers by design: no octal escapes, and decimal escapes are always treated as backreferences.
The "v" (unicodeSets) flag is not supported by the validation engine — use "u" for Unicode property patterns.
The test string is capped at 50,000 characters; the matches list shows the first 500 matches while the reported count is always exact.
Matching uses your JavaScript engine's native RegExp, so results match real JS behavior — including any catastrophic-backtracking slowness of pathological patterns.
This version validates and tests patterns but does not render a railroad diagram: the vendored regulex build ships no SVG visualizer (its rewrite is incomplete upstream).

How it works

Syntax is validated by regulex (vendored open-source JavaScript regex parser, MIT), which reports precise typed errors with the exact position — e.g. ParenMismatch for an unclosed group. Matching itself runs on your JavaScript engine's native RegExp, so results are identical to what the pattern does in real JS code. The test string is capped at 50,000 characters; the matches list shows the first 500 matches while the count stays exact. Everything runs in your browser — nothing is uploaded.

Examples

Find phone-like fragments

Status: ok — 2 matches: "555-1234", "555-9876".

Invalid pattern shows the exact position

Status: error — regulex reports ParenMismatch (unbalanced parenthesis) at position 1.

Case-insensitive matching

Status: ok — 3 matches.

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