Regex tester (JavaScript)

How to use

1. Type the pattern without the surrounding slashes and choose flags such as g, i and m.
2. Enter the test string. Matches are highlighted, and the count, index and groups of each match are listed.
3. To try a replacement, turn on "Show replace preview" and enter a replacement string ($1, $<name>, $& and so on).
4. Under "Use it in code", pick JavaScript, Python or PHP to copy a snippet that uses the pattern.
5. Pick a sample (dates, email addresses, URLs and more) to see an example.

Notes and limits

• The engine is the JavaScript (ECMAScript) RegExp of your browser. PCRE (PHP), Python, Java, .NET and other engines differ in syntax and behavior, so a result here is not guaranteed to be the same elsewhere.
• The v flag is offered only when your browser supports it.
• A search that does not finish within 1.5 seconds is stopped (protection against catastrophic backtracking from patterns such as (a+)+$). Safari (JavaScriptCore) may give up on such a search by itself and report no match.
• Test strings up to 200,000 characters. Up to 10,000 matches are collected; the list shows the first 200 and the first 1,000 are highlighted.
• Indexes are counted in UTF-16 units, as in JavaScript, so an emoji can count as 2.
• Code snippets are a starting point. The Python and PHP versions convert syntax such as named groups, but not every difference.

How it works

Matching runs in a Web Worker, separate from the page. If it takes too long, the worker is terminated, so the page never freezes. "Cancel" stops it at any time.

With the g flag every match is found; without it only the first. After an empty match the search moves on by one character (one code point with u or v).

Group positions come from the d flag (hasIndices) and are drawn nested inside each match. Nothing is inserted as HTML; all text is shown as plain text.