Regex Tester — Test & Debug Regular Expressions Online

This free regex tester lets you build and debug regular expressions against your own text, highlighting matches in real time. It uses the JavaScript RegExp engine and supports flags for global, case-insensitive, multiline, and dotall matching, plus capture groups and named groups. It helps you reason about greedy versus lazy quantifiers — the most common source of over-matching — so your pattern catches exactly what you intend. Test against valid and deliberately malformed input. Everything runs in your browser, so your text is never uploaded.

Enter a pattern above
Matches highlighted
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How the regex tester works

Regular expressions (regex) are patterns used to match, search, and manipulate text. This tester uses JavaScript's native RegExp constructor, making it directly compatible with any JavaScript code. Enter your pattern (without delimiters) and optional flags in the dedicated field.

Key flags: g (global — find all matches, not just first), i (case-insensitive), m (multiline — ^ and $ match line starts/ends), s (dotAll — . matches newlines too). Common patterns: \d+ matches one or more digits, \w+ matches word characters, ^ anchors to line start, $ to line end, [a-z] matches a character range, (group) captures a group. Matches are highlighted in the test string and all capture groups are shown. The tester is client-side — no input leaves your browser, making it safe to test patterns against sensitive data.

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Related Guide: Learn more about Regex Performance

Greedy vs lazy matching

By default quantifiers like *, + and {n,} are greedy — they grab as much text as possible, then backtrack. Add a ? to make them lazy, matching as little as possible. The classic trap is <.*> on <a><b>: greedy matches the whole string in one go, while <.*?> correctly matches each tag. Knowing which mode you want prevents most "why did it match too much?" surprises.

Flags that change everything

Anchors and groups

Anchor patterns with ^ and $ to match whole strings (vital for validation), use \b for word boundaries, and prefer non-capturing groups (?:...) when you only need grouping, not extraction. Test against both valid and deliberately malformed inputs — a regex that accepts good data but also accepts garbage is the most common validation failure.

⚠️ Common Mistakes to Avoid

Frequently asked questions

What is Regex?

Regular Expressions (Regex) are sequences of characters that define search patterns for text processing.

Do I need to learn regex?

It is a powerful tool for developers and data analysts for searching, replacing, and validating strings.

Related guides

Regex Performance & Backtracking → Regex vs Wildcards →
Reviewed by the ToolsmithPro editorial team · Last updated June 2026. Every calculation and conversion runs entirely in your browser — your inputs are never uploaded, stored or shared. Formulas and methodology are documented on our about page; spot an error? tell us and we'll fix it.