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Regex Tester

Live JavaScript regex tester with match highlighting and flags.

Best for: Validating input such as emails or phone numbers, extracting fields from text, building find-and-replace patterns, and learning how a tricky expression behaves.

Complete guide

Input

//
3 matches
[0] Hello
[6] world
[12] 123
How it's calculated & sources
The Regex Tester uses the standard, deterministic formula, so the same inputs always produce the same result. It runs entirely in your browser, so your inputs are never uploaded. Last reviewed June 2026.

Free & no sign-up · runs entirely in your browser. Results are estimates for general information, not professional advice — verify important decisions with a qualified expert. Last reviewed June 2026.

How it works

Uses the native JavaScript RegExp engine. Flags: g global, i ignore case, m multiline, s dotAll, u unicode.

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Complete guide

Quick answers

Short, sourced answers to the questions people (and AI assistants) ask most.

What is Regex Tester?
A regex tester runs a regular expression against sample text in real time, highlighting matches and capture groups so you can build, debug and understand a pattern before pasting it into code.
Why does Regex Tester matter?
Highlighted spans show exactly what the pattern captures; no match means the pattern, as written, does not apply to the text — usually an escaping, anchor or quantifier issue.
How is Regex Tester calculated?
It compiles your pattern with the flags you pick — g (global), i (case-insensitive), m (multiline), s (dotAll), u (unicode) — and runs it against your test string with the JavaScript regular-expression engine, then shows each match and the contents of every capture group.
What are common regex tester mistakes?
Catastrophic backtracking from nested quantifiers like (a+)+ on long input — it can hang the engine.
When should you use the Regex Tester?
Validating input such as emails or phone numbers, extracting fields from text, building find-and-replace patterns, and learning how a tricky expression behaves.

What is the Regex Tester?

A regex tester runs a regular expression against sample text in real time, highlighting matches and capture groups so you can build, debug and understand a pattern before pasting it into code.

How the Regex Tester works

It compiles your pattern with the flags you pick — g (global), i (case-insensitive), m (multiline), s (dotAll), u (unicode) — and runs it against your test string with the JavaScript regular-expression engine, then shows each match and the contents of every capture group.

Common regex tokens

\d \w \sA digit, word char, or whitespace
.Any character (except newline, unless s flag)
* + ?0+, 1+, or 0-1 of the previous item
{2,4}Between 2 and 4 repetitions
^ $Start / end of line or string
( ) (?: )Capturing group / non-capturing group
\bA word boundary

Key terms

Capture group:
Parentheses that remember the matched text for reuse or extraction.
Quantifier:
How many times the previous item may repeat (* + ? {n,m}).
Anchor:
A zero-width assertion such as ^, $ or \b that fixes a position.

Common mistakes to avoid

  • Catastrophic backtracking from nested quantifiers like (a+)+ on long input — it can hang the engine.
  • Forgetting to escape special characters: . * + ? ( ) [ ] { } ^ $ | \ /.
  • Using regex to parse HTML, JSON or other nested formats — use a real parser instead.
  • Greedy vs lazy: .* grabs as much as possible; use .*? when you want the shortest match.
  • Omitting anchors (^ $ \b) so the pattern matches more than you intended.

Detailed FAQ

Why does my pattern match too much?

Quantifiers are greedy by default. Add ? to make them lazy (e.g. <.*?>), or anchor the pattern so it cannot wander.

Are regex flavours the same everywhere?

No. This tester uses JavaScript's engine; PCRE (PHP), Python's re and Java differ in lookbehind, named groups and some escapes, so test in your target language too.

Keywords: regex, regexp, pattern.

Sources: MDN — Regular expressions

Reviewed by the Free Tools Galaxy editorial team · Updated June 2026 · Calculated privately in your browser.

Frequently asked questions

Is the Regex Tester free to use?+

Yes. Every tool on Free Tools Galaxy is 100% free, runs in your browser and requires no signup.

How accurate is the Regex Tester?+

Regex Tester uses the standard regex formula in double-precision arithmetic, so the same inputs always produce the same result and you can verify any figure by hand. It is an educational estimate — real-world outcomes depend on your actual rates, rules and assumptions.

Do you store my inputs?+

No. The Regex Tester runs entirely in your browser. Nothing is uploaded or saved to a server.

Can I use the Regex Tester on mobile?+

Yes — the interface is fully responsive and works on phones, tablets and desktops.

What are common mistakes to avoid?+

The most frequent mistake is mixing units. Double-check your inputs use a single, consistent unit before clicking Calculate.

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Title: I built/used this free regex tester — instant, no signup

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Regex Tester on Free Tools Galaxy — Live JavaScript regex tester with match highlighting and flags. https://freetoolsgalaxy.com/tools/regex-tester
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# Regex Tester — the free tool I keep coming back to

Live JavaScript regex tester with match highlighting and flags.

Try it: https://freetoolsgalaxy.com/tools/regex-tester
Quora answer
The fastest way is to use a free in-browser regex tester: https://freetoolsgalaxy.com/tools/regex-tester. Live JavaScript regex tester with match highlighting and flags.

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