JavaScript Regular Expression Tester

This utility evaluates regular expressions directly against your browser’s native JavaScript RegExp runtime, ensuring that every pattern behaves identically to production Node.js, Cloudflare Workers, Deno, and modern browser environments.

1. Enter a Regular Expression Pattern: Type your expression between the forward slashes (/.../) without enclosing delimiters, or click one of the Pattern Presets above to load a working example.
2. Configure Expression Flags: Toggle flags such as g (global), i (case-insensitive), m (multiline), s (dotAll), u (Unicode), or d (capture group indices) using the interactive flag bar.
3. Inspect Matches & Capture Groups: Paste your multiline sample text. Matched substrings are highlighted immediately, and the Matched Groups & Positions panel displays the exact character start/end indices, line/column numbers, numbered sub-groups ($1, $2), and named capture groups (match.groups).
4. Test Replacements & Export Code: Switch to the String.replace() Preview tab to test substitution tokens ($&, $1, $<name>), or use the top toolbar buttons to copy the regex literal, ready-to-run JavaScript snippet, or structured JSON match array.

Need a deeper syntax walkthrough? Read our companion JavaScript Regex Guide: Syntax, Examples & Patterns.

Below are battle-tested regular expressions frequently used in frontend validation, server-side routing, and log parsing. You can load most of these directly using the preset buttons inside the tester above:

Modern JavaScript engines support eight regular expression flags that modify how patterns are compiled and executed:

Parentheses in JavaScript regular expressions serve two distinct purposes: grouping quantifiers/alternations and extracting matched sub-strings for inspection or replacement.

const logLine = "2026-10-12T22:45:10Z [ERROR] Worker timeout after 1500ms";
const pattern = /^(?<timestamp>\S+) \[(?<level>INFO|WARN|ERROR)\] (?<message>.+?) \((?<ms>\d+)ms\)|^(?<ts>\S+) \[(?<lvl>INFO|WARN|ERROR)\] (?<msg>.+)$/;

const urlPattern = /(?<protocol>https?):\/\/(?<host>[^/]+)(?<path>\/.*)?/d;
const match = urlPattern.exec("https://www.webdevawards.org/tools/regex-tester");

if (match) {
  console.log(match[0]);              // Full match
  console.log(match[1]);              // "https" (Numbered Group $1)
  console.log(match.groups.host);     // "www.webdevawards.org" (Named Group)
  console.log(match.indices.groups);  // { protocol: [0, 5], host: [8, 28], path: [28, 47] }
}

• Capturing Group (pattern): Captures the matched substring into numbered slots (match[1], match[2]) and allows backreferences inside the pattern via \1, \2.
• Named Capturing Group (?<name>pattern): Assigns a semantic identifier accessible on match.groups.name, backreferenced via \k<name>, and referenced in replace() via $<name>.
• Non-Capturing Group (?:pattern): Groups tokens for quantifiers or alternation (e.g., (?:http|https)) without allocating memory in the capture array.
• Lookahead & Lookbehind Assertions: Zero-width assertions that verify surrounding text without consuming characters: positive lookahead (?=...), negative lookahead (?!...), positive lookbehind (?<=...), and negative lookbehind (?<!...).

In February 2007, software developer Brendon Crawford launched REJAX at http://www.projects.aphexcreations.net/rejax/ (and projects.aphexcreations.net/rejax) under the Aphex Creations banner. Described across developer bookmarking communities as “rejax - easiest regular expression tester. ever.” and “rejax - coolest regular expression tester. ever.”, REJAX was one of the first developer tools to demonstrate that complex regular expression debugging could run entirely inside the browser DOM without server-side round-trips.

Over nearly two decades, REJAX was cited by Ajaxian, phpspot, KickJava, Xmarks, Diigo JavaScript Online Tools, and the Gouvernement du Québec WebMaestro portal (as a reference testeur d’expressions régulières). When the historical Aphex Creations domain was retired and migrated into the Web Dev Awards developer resources archive, preserving a fast, zero-advertisement, interactive JavaScript regular expression tester at this destination was our top priority so every historical link continues to serve working code.