Regex Tester

Frequently asked questions

Is my pattern or test text uploaded anywhere?

No. Every pattern is evaluated entirely in your browser — nothing you type is sent to a server or logged.

Which regex engine does this use?

The JavaScript (ECMAScript) regex engine, via the browser's built-in RegExp. Syntax and behavior can differ from Python, PCRE, .NET, or other regex dialects — a pattern that works here may need adjustment elsewhere.

Why did evaluation stop with a timeout message?

Pattern evaluation runs in a background worker with a timeout. If a pattern causes catastrophic backtracking (exponential-time matching on certain inputs), the worker is terminated and you're shown a timeout message instead of freezing the page.

Why are matches capped at 1000?

To keep results readable and the page responsive, at most 1000 matches are enumerated. If your pattern matches more than that, the results are marked as truncated.

Are the sample patterns production-ready?

No — the built-in samples (like the email-like or URL patterns) are simplified demonstrations. They may not be sufficient for security-sensitive validation; use a well-tested library for cases like real email or URL validation.

How accurate is the plain-English pattern explanation?

It's a best-effort, token-by-token explanation meant to help you read a pattern faster — not a full regex parser. For unusual or deeply nested syntax, treat it as a starting point rather than an authoritative breakdown.

What's the difference between greedy and lazy quantifiers?

Greedy quantifiers (*, +, ?, {n,m}) match as much text as possible before backtracking; their lazy counterparts (*?, +?, ??) match as little as possible. Both are supported here exactly as the JavaScript engine implements them.

You might also need

What is the Regex Tester?

The Regex Tester lets you build and debug JavaScript regular expressions against real text entirely in your browser. Enter a pattern and flags, paste the text you want to match, and see every match, capture group, and named group highlighted instantly — with an optional replace-mode preview and a plain-English, best-effort breakdown of the pattern.

Use cases

• Debugging a regex used in application code or a validation rule
• Extracting structured data (dates, log fields, IDs) from pasted text
• Previewing a find-and-replace before running it in an editor or script
• Learning what a pattern does via the best-effort plain-English explanation

How to use the Regex Tester

1. 1

   Enter a pattern and flags

   Type a regular expression and toggle the flags you need (g, i, m, s, u, y, d).

2. 2

   Paste your test text

   Paste the text to test against — matches highlight live as you type.

3. 3

   Review matches or preview a replacement

   Inspect each match's groups, or switch on Replace mode to preview a substitution before using it elsewhere.

Examples

Created 2026-01-15, updated 2026-09-10.

2026-01-15, 2026-09-10

2026-09-10 → $<month>/$<day>/$<year>

09/10/2026