🔍 Regex Tester

The Regex Tester lets you build and check a JavaScript regular expression against sample text, highlighting every match as you type. Regular expressions are powerful but easy to get subtly wrong, and seeing exactly what your pattern matches (and what it misses) is the fastest way to get it right.

How to Use This Tool

Enter your pattern and any flags (g, i, m, s, u), then paste some test text. Matches are highlighted in the text and counted. An invalid pattern shows the engine's error message so you can correct it.

What This Tool Checks

It uses your browser's native JavaScript regex engine — the same implementation Node.js uses — so behaviour matches what you will get in JavaScript code. The "g" flag finds all matches; without it you get only the first.

Understanding Your Results

The highlight shows precisely which spans of text your pattern captures. A pattern that matches too much (greedy quantifiers) or too little (a missing character class) becomes obvious immediately. The match count helps confirm you are catching every occurrence.

How to Fix Common Problems

If nothing matches, check for an accidental anchor (^ or $), an unescaped special character (. + * ? all have meaning), or a missing "g" flag when you expected multiple matches. If everything matches, a greedy .* is the usual cause.

SEO Best Practices

Build patterns incrementally, testing after each addition. Escape literal special characters. Remember regex flavours differ — a pattern from a PHP or Python tutorial may not behave identically here, since this uses the JavaScript engine. Avoid regex for parsing HTML or nested structures.

Example

The pattern \b\w+@\w+\.\w+\b with the g flag highlights simple email-like tokens in a block of text, letting you confirm it catches "a@b.com" while ignoring ordinary words.

Common Mistakes to Avoid

Forgetting the g flag and wondering why only the first match highlights. Using a pattern written for a different regex flavour without adjusting it. Reaching for regex to parse HTML, which it cannot do reliably.