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Regex Tester

Text & Code · Free Online Tool

Test regex with live match highlighting.

By CoderNewbie TeamLast updated: 2026-09-20

//g

Flags:

2 matches found

Test Text76 chars

Highlighted Matches2 matches

Match Details

#1pos: 14"[email protected]"groups: [hello, example.com]

#2pos: 35"[email protected]"groups: [support, ez4code.dev]

Live Highlighting

See matches highlighted in real time as you type your pattern and test text.

Capture Groups

Inspect every capture group for each match, with position and value laid out clearly.

Flags Support

Toggle g, i, m, and s flags to fine-tune your pattern.

How to Use Regex Tester

1Enter or paste your text or code into the input area.
2Choose the desired operation (format, convert, analyze, etc.).
3Click the action button to process.
4Copy the result from the output area.

Features

✓100% free — no signup, no subscription, no ads
✓Runs entirely in your browser — data never leaves your device
✓Fast and lightweight — instant results
✓Cross-platform — works on desktop, tablet, and mobile

What is Regex Tester?

Regular expressions (regex) are a pattern-matching language defined by IEEE POSIX and ECMAScript specifications. They describe search patterns using metacharacters and literals. A Regex Tester lets you evaluate patterns against test strings in real time, highlighting matches and capturing groups.

How does Regex Tester work?

The tester compiles your pattern into a state machine (NFA/DFA) and applies it to the input string. Flags modify behavior: g (global), i (case-insensitive), m (multiline), s (dotAll), u (unicode), y (sticky). Match results show position, captured groups, and named groups.

Common Use Cases

✓Form validation — emails, phone numbers, postal codes
✓Log parsing — extract fields from structured log lines
✓URL routing — match path patterns in web frameworks
✓Search and replace — batch text transformations

Regex vs String Methods

String methods (includes, indexOf, startsWith) are simpler and faster for exact matches. Regex is necessary for pattern-based matching (e.g., 'all emails in a document'). For simple checks, prefer string methods; for flexible patterns, use regex. Overly complex regex can cause ReDoS (Regular Expression Denial of Service).

Security & Privacy

This tool runs entirely in your browser. Beware of ReDoS when using regex in production — certain patterns (e.g., nested quantifiers like (a+)+) can cause catastrophic backtracking. Use timeouts or safe regex libraries for user-supplied patterns.

Technical Details

Specifications: ECMAScript 2024 (MDN Reference), IEEE POSIX. Metacharacters: . * + ? ^ $ { } [ ] \ | ( ). Character classes: \d \w \s \b. Quantifiers: * (0+), + (1+), ? (0-1), {n,m}. Lookahead: (?=...) (?!...). Lookbehind: (?<=...) (?<!...). Named groups: (?<name>...).

Frequently Asked Questions

What is the difference between greedy and lazy quantifiers?

Greedy quantifiers (*, +) match as much as possible; lazy quantifiers (*?, +?) match as little as possible. Use lazy when you want the shortest match.

How do I test for a whole word?

Use word boundaries: \bword\b matches 'word' but not 'sword' or 'words'.

What are lookahead and lookbehind?

Lookahead (?=...) asserts what follows without consuming it. Lookbehind (?<=...) asserts what precedes. They are zero-width assertions useful for conditional matching.