Regex Tester

Interactive Studio

Test, debug, and explain regular expressions with live match overlays, capture groups inspector, replacement simulator, and cheat sheets. Instant feedback with zero server storage.

Patterns:

Regular Expression

/g/

Flags & Modifiers

Test Corpus String98 chars

Highlighted Match Overlay2 matches found

Contact support@toolium.dev#1 or hello@company.co.uk#2 for inquiries. Avoid invalid emails like user@.

Total Matches

Match Status

Matched

Active Flags

/g/

Loading tool…

About Regex Tester

A regular expression (RegEx) is a powerful pattern syntax for matching and extracting strings. This studio provides a real-time testing environment with instant syntax validation, granular flag modifiers (g, i, m, s, u), interactive match highlighting badges, structured capture group tables, and a regex replacement engine with backreference support.

How to use Regex Tester

Enter the pattern without surrounding slashes.
Toggle flags such as g, i, m, s and u.
Paste test text. Matches are highlighted and capture groups are listed.
Use the replace field with $1-style backreferences to preview substitutions.

Worked examples

Input: Pattern (\d{4})-(\d{2}) on text 2026-10

Result: Match 2026-10 with group 1 = 2026 and group 2 = 10.

Input: Pattern (\w+)@(\w+)\.com replaced with $2: $1 on ada@byteleaps.com

Result: byteleaps: ada

Background

This tester uses the JavaScript regular expression engine, so syntax and flags match JavaScript. Other engines (PCRE, Python, Java, .NET) differ in lookbehind support, named groups and flag letters.

Quantifiers are greedy by default and match as much as possible. Add ? to make them lazy. Patterns with nested quantifiers such as (a+)+ can take exponential time on certain inputs, known as catastrophic backtracking.

Common mistakes

Forgetting the g flag and wondering why only the first match shows.
Using a greedy .* that swallows more text than intended.
Not escaping special characters such as . or ( when you mean them literally.

Frequently Asked Questions

Is my regex evaluated locally?

Yes, all regular expressions are executed locally using your browser's native JavaScript RegExp engine with zero network requests.

How do capture groups work?

Enclose sub-patterns in parentheses e.g. `(\d{4})-(\d{2})`. The Capture Groups tab will display each numbered match group along with its character boundaries.

Can I test regex string replacement?

Yes! Switch to the 'Replace' tab, enter your replacement string (with `$1`, `$2` or `$&` backreferences), and inspect the live replaced text output.

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