Regex Tester

How to use it

1. Type the pattern without the slashes. The example finds dates written as 2026-11-10.
2. Tick the flags you need. g finds every match, i ignores upper and lower case, and m and s change how line breaks are read.
3. Paste your test text. The matches are highlighted at once, and the list under the text gives the position and groups of each one.
4. Type a text in Replace with to see the result, then press Copy to take it.

How the matching works here

The page uses the regular expression engine built into your browser, the one JavaScript uses. Matches are found in order, from the start of the text. The position shown is the count of characters before the match, starting at 0, counted in UTF-16 units as JavaScript counts text.

The search runs in a background worker of your browser, so the page stays responsive. A search stops after 10,000 matches, and a text longer than 200,000 characters is not searched. A pattern that can match nothing, such as x*, moves on by one character after each empty match, so the search always ends. A search that takes longer than 2 seconds is stopped; see the section on catastrophic backtracking.

• Without the g flag, only the first match is shown, as String.match() does.
• The u flag reads an emoji or any other character outside the basic plane as one character.
• The d flag adds the start and end of each group to the list.

Catastrophic backtracking and the 2-second limit

A regular expression engine tries the ways a pattern could match, one after another. With some patterns, mostly a repeated group inside another repeated group such as (a+)+, the number of ways grows very fast: each extra character can double the work. This happens most when the text almost matches and fails only at the very end.

For example, the pattern (a+)+$ on a text of many letters a followed by an exclamation mark cannot match, because $ needs the end of the text. The engine still tries every way of splitting the letters before it gives up. With about 26 letters the search can already take a second or more on a typical computer, and each letter more makes it much slower. The time depends on your device.

This page runs each search in a background worker and stops it after 2 seconds, showing a message. The rest of the page keeps working, so you can change the pattern or the text and try again.

• Do not put a repeat inside another repeat over the same characters: write (a+)$ instead of (a+)+$. On the same text the safe pattern finishes at once.
• Be specific: use [a-z] or \d+ inside a group instead of a dot, so that the parts of a match cannot overlap in many ways.
• Test the pattern on a short sample of the text first, then on the full text.
• Anchor the pattern where you can: ^ at the start and $ at the end stop the engine from trying every starting point.

Replacement patterns

The replacement text follows the rules of String.replace() in JavaScript. A dollar sign starts a code that is replaced by the matched text:

Differences from PCRE and .NET

A pattern written for PCRE (PHP, many Linux tools) or for .NET (C#, Visual Basic) usually works here, but some features exist only in one of the engines. The table lists the ones that matter most.

Frequently asked questions