Bitwise Calculator
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Regex Tester
Test a regular expression live, with every match and capture group shown.
Quick Tool Overview
The Regex Tester runs a pattern against your own text on every keystroke, highlighting matches in place and listing every capture group — with a guard against the catastrophic backtracking that can otherwise lock the page.
- What it does
- Tests a JavaScript regular expression live
- You give it
- A pattern, flags and some test text
- You get back
- Highlighted matches, groups and a replace preview
- Best for
- Writing and debugging a pattern before it ships
About the Regex Tester
Type a pattern and it runs against your text on every keystroke, with matches highlighted in place so you can see exactly what is and is not being caught. Every match is also listed with its position and each of its capture groups.
Named groups are shown by name, which matters once a pattern has more than two of them and counting brackets stops being reliable. There is a replace mode that previews the result with $1 and named references working as they would in code, and a catastrophic-backtracking guard that stops a runaway pattern from locking the page — which is a real risk with nested quantifiers and the reason some regular expressions take down production servers.
Key features of the Regex Tester
Matches highlighted in place
Seeing what is caught inside the real text is far more useful than a list of strings out of context.
Named groups by name
Once a pattern has three groups, counting brackets stops being reliable — so named groups are listed by their names.
Replace preview
$1 and named references behave exactly as they will in your code, so the replacement is checked rather than assumed.
Backtracking guard
A pattern like (a+)+b can take exponential time on near-matching input. The tester stops rather than freezing — which is also a real production risk worth knowing about.
Flags explained
What g, i, m, s and u each change, where you are turning them on.
How to use the Regex Tester
-
1
Write your pattern
Without the delimiters; the flags have their own controls.
-
2
Paste your test text
Matches are highlighted in place as you type.
-
3
Check the groups
Every match lists its position and each capture group, named or numbered.
-
4
Try a replacement
Preview the result with $1 and named references working as in code.
Frequently asked questions
JavaScript, since it runs in your browser. It is close to PCRE for everyday patterns, but lookbehind, named groups and Unicode property escapes differ in support — and JavaScript has no possessive quantifiers or atomic groups.
It finds every match rather than stopping at the first. Without it, replace only replaces once and match returns a single result with its groups. It is the flag people forget most often.
A pattern with nested quantifiers, such as (a+)+b, can take exponential time on input that nearly matches. It is a genuine denial-of-service vector, and the tester guards against it rather than freezing.
The m flag makes ^ and $ match at each line rather than only at the ends of the string. The s flag makes the dot match newlines too. They are different things and are often both needed.
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