Regex Tester

Build and debug regular expressions against your own test text and watch the matches highlight as you type. This Regex Tester uses the JavaScript (ECMAScript) regex engine, shows each match with its index and capture groups, and lets you preview a find-and-replace. Everything runs locally in your browser.

Regular Expression

Flags

Test String

Matches (highlighted)

Hello World
Foo123
Bar

Test replacement

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Why use Regex Tester?

Regular expressions are concise but unforgiving, one misplaced quantifier changes everything an expression matches. The fastest way to get them right is to test interactively against real input, which is exactly what this tool does: as you edit the pattern or the test string, matches re-highlight immediately and a running count tells you how many were found. The Match Details panel is where debugging happens. It lists every match, the index where it starts, and the contents of each capture group, so you can confirm not just that something matched, but that the right parts were captured. The replacement preview then lets you see the result of a substitution, with $1 and $2 standing in for your groups, before you run it for real. One practical caution: this is the JavaScript regex flavor. It is perfect for front-end and Node work, but if you will ultimately run the expression in Python, PCRE, or another engine, double-check any advanced features there, since lookbehind support and named-group syntax are not identical across flavors.

Key benefits

Live highlighting shows exactly what your pattern matches
Per-match details include the start index and captured groups
Built-in replacement preview with $1/$2 group references
Immediate, readable error messages for invalid patterns
Client-side, so your test data stays on your machine

Features

JavaScript (ECMAScript) regex flavor, matching what runs in Node and browsers
Supports g, i, m, s, u, and y flags
Match count and index reporting
Find-and-replace preview

Popular use cases

Writing a validation pattern for emails, slugs, or phone formats
Extracting capture groups from log lines or CSV-like text
Testing a search-and-replace before running it across a codebase
Learning how flags like multiline and dotall change matching

How to use Regex Tester

1
Enter your pattern
Type the regular expression into the "Regular Expression" field (no slashes needed).
2
Set flags
Add flags such as g (global), i (case-insensitive), m (multiline), or s (dotall) in the Flags box.
3
Add test text
Paste the string you want to match against; matches highlight instantly and a match count appears.
4
Inspect matches
Review the Match Details list to see each match, its index, and any capture groups.
5
Preview a replacement
Tick "Test replacement" and enter a replacement string, using $1, $2 to reference capture groups.

Frequently asked questions

Which regex flavor does this use?

It uses the JavaScript (ECMAScript) engine, the same one in browsers and Node.js. If you are targeting PCRE, Python, or Go, some syntax (notably certain lookbehind and named-group features) can differ, so verify in your target language.

What do the flags mean?

g matches all occurrences, i ignores case, m makes ^ and $ match line boundaries, s lets . match newlines, u enables full Unicode, and y anchors matching to the lastIndex position.

How do I use capture groups in the replacement?

Reference them with $1, $2, and so on, in the replacement field. $1 is the first parenthesized group. Enable "Test replacement" to preview the output.

Why does my pattern show an error?

The engine reports invalid syntax such as an unbalanced bracket or parenthesis, or an unsupported escape. The exact message appears above the test area so you can fix it.

Is my test data private?

Yes. Matching runs entirely in your browser; nothing you type is uploaded.

Last reviewed: July 2026

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