Which regex flavor is supported?

JavaScript regular expressions (the same engine your browser uses via the native RegExp object), including the g, i, m and s flags. Patterns valid in JS are valid here.

Does the explainer cover everything?

It explains the common metacharacters, character classes, quantifiers, anchors and groups token by token. It is a learning aid, not a full formal parser, so very exotic constructs may be summarised rather than fully decomposed.

Yes. Everything runs in your browser — your pattern and test text are never sent anywhere.

What do the g, i, m and s flags do?

g (global) finds every match instead of just the first; i (ignore case) makes the match case-insensitive; m (multiline) makes ^ and $ match at line breaks; s (dotall) lets the dot . also match newline characters.

Why are only the first matches highlighted without the g flag?

For highlighting, the tester always forces the global flag internally so you can see every match. In real code, a regex without g stops after the first match — so remember to add g when you need all matches.

How do I match a literal special character like a dot or bracket?

Escape it with a backslash. Use \\. for a literal full stop, \\( for a literal parenthesis, and \\\\ for a literal backslash. Unescaped, those characters have special meaning.

What is the difference between greedy and lazy quantifiers?

Greedy quantifiers like .* match as much as possible; adding a ? makes them lazy (.*?) so they match as little as possible. Lazy quantifiers are the fix when .* swallows too much text.

Regex Tester with Plain-English Explainer

Test regular expressions and understand them

This tester highlights every match live as you type and gives a plain-English, step-by-step explanation of what each part of your pattern does — something most regex tools skip. It is built for developers writing and debugging patterns, and for anyone learning regex who wants to know why a pattern matches.

How it works

Your pattern and flags are compiled with the browser’s native new RegExp(pattern, flags) — the exact JavaScript engine your code will use, so behaviour matches production. For highlighting, the tester forces the global flag (g) internally and runs test.replace(regex, ...) to wrap every match in a highlight, even if you didn’t type g. If the pattern is invalid, the RegExp constructor’s error is shown instead. Separately, a lightweight tokeniser walks the pattern left to right and emits a sentence per token (character class, quantifier, anchor, group, alternation, and so on) for the explanation panel.

Example

Pattern \b\d{4}-\d{2}-\d{2}\b with the g flag, tested against:

Invoices dated 2026-05-30 and 2026-06-01 are due.

highlights both 2026-05-30 and 2026-06-01. The explainer breaks it down as: word boundary, four digits, a literal hyphen, two digits, a hyphen, two digits, word boundary — i.e. an ISO-style date.

Flag reference

Flag	Name	Effect
`g`	global	Match every occurrence, not just the first
`i`	ignore case	Case-insensitive matching
`m`	multiline	`^` and `$` match at each line break
`s`	dotall	`.` also matches newline characters

Everything runs locally in your browser — your pattern and text are never uploaded.