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Regex Tester

Test regular expressions with real-time matching, capture groups, and flag support.

Pattern
Flags
Test string
Matches

100% client-side · No signup · Free forever. This Dev tool runs entirely in your browser — nothing is uploaded.

A regex tester is one of the highest-leverage tools in any text-processing workflow: writing log parsers, validating user input, scraping HTML, building search-and-replace queries, or just sanity-checking a complex pattern before deploying it. This regex tester runs entirely in your browser using the JavaScript ECMAScript regex engine — the same engine that powers String.prototype.match(), RegExp.prototype.test(), and the regex features in Node.js, every modern browser, and many JavaScript-adjacent tools.

How the Regex Tester Works

Type or paste a regex pattern (without the surrounding slashes), specify the flags (g, i, m, s, u, y), and a test string. The tester runs String.prototype.matchAll() against your input with the pattern compiled at every keystroke, highlighting each match and showing capture groups. Invalid patterns (unbalanced parens, bad escape sequences, undefined backreferences) produce a parse error with the exact problem so you can fix it incrementally rather than waiting for runtime.

When to Use a Regex Tester

  • Input validation — email addresses, phone numbers, URLs, postal codes, credit card formats.
  • Log parsing — extracting timestamps, IPs, error codes from unstructured logs.
  • Search and replace — sed/awk/VS Code/IntelliJ regex search before running destructive replaces.
  • Web scraping — pulling specific fields from HTML or text responses.
  • Data cleaning — normalizing phone formats, stripping HTML tags, collapsing whitespace.
  • URL routing — Express, Flask, FastAPI all support regex route patterns.
  • Learning regex — build patterns incrementally with instant feedback.

Regex in Code

// JavaScript
const re = /\b(\w+@\w+\.\w+)\b/g;
const matches = [...text.matchAll(re)];
matches.forEach(m => console.log(m[0], 'at', m.index));

// Python
import re
matches = re.finditer(r'\b(\w+@\w+\.\w+)\b', text)

// CLI
grep -oE '[a-z]+@[a-z]+\.[a-z]+' file.txt
ripgrep / rg: rg -o '\b\w+@\w+\.\w+\b' file.txt

Regex Flavor Compatibility

This tester uses the ECMAScript regex flavor — the same one JavaScript, TypeScript, Node.js, Deno, and the browser use natively. ECMAScript regex is broadly similar to PCRE (PHP, Perl), Python's re module, and POSIX extended regex, but with some differences: lookbehind support arrived in ES2018; named capture groups use (?<name>...) syntax (same as Python); the u flag enables full Unicode support including surrogate pairs and Unicode property escapes (\p{Letter}). If a pattern works here, it works in any modern JavaScript runtime. For Go regex (RE2), watch out for unsupported features like backreferences and lookahead.

Frequently Asked Questions

Everything you need to know about the Regex Tester.

Which regex flavour is used?

The standard JavaScript RegExp engine (ECMAScript). Flags like g, i, m, s, u, y are all supported.