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Regex Explainer

Free regex explainer. Paste any regular expression to break it down token by token and read a plain-English explanation of each literal, class, anchor, group, quantifier and flag. Runs in your browser.

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Regular expression

Flags

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Enter a regex to break it down.

Deep dive guides

A LITTLE CONTEXT · 3 MIN READHow to Read a Regular Expression, One Token at a Time

All 3 related guides

How Regular Expressions Work — The regex vocabulary — character classes, quantifiers, anchors, groups — and three everyday patterns explained token by token.
How Email Validation Actually Works — The anatomy of an email address, the one regex browsers use for input type=email, what it allows and rejects, and why syntax checks are not delivery checks.
How to Read a Regular Expression, One Token at a Time — Regex looks like line noise until you read it as tokens: literals, classes, anchors, groups and quantifiers, each with a plain-English meaning.

How regex explainer works

A regular expression is a string of tokens that a regex engine interprets as a search pattern. This explainer walks your pattern from left to right and splits it into its constituent tokens — literal characters, escapes like \d and \w, character classes in square brackets, anchors like ^ and $, groups, quantifiers like * and {2,4}, and the alternation |.

Each token is labelled with its type and given a one-line plain-English description, so a pattern such as ^[A-Z]\w+@\w+\.\w+$ reads as "start of string, an uppercase letter, one or more word characters, an @, one or more word characters, a dot, one or more word characters, end of string." Quantifiers report whether they are greedy or lazy and their min and max.

The pattern is first validated by constructing a RegExp from it, so an unbalanced parenthesis or invalid escape returns a clear error instead of a partial explanation. The flags are decoded too (g global, i case-insensitive, m multiline, s dotAll, u unicode, y sticky). Everything runs in your browser.

Frequently asked questions

How is this different from the regex tester?

The regex tester runs your pattern against a string and shows the matches. This explainer does not run the pattern — it parses the pattern itself and explains what each token means in plain English, which helps you read a regex you did not write.

Does it support lookbehind and named groups?

Yes. It recognises capturing and non-capturing groups, positive and negative lookahead, positive and negative lookbehind, and named capturing groups using the (?<name>…) syntax, labelling each accordingly.

What are greedy and lazy quantifiers?

A greedy quantifier (*, +, ?, {n,m}) matches as much as possible. Adding a trailing ? makes it lazy (non-greedy), so it matches as little as possible. The explainer flags which one each quantifier is.

Why does it say my regex is invalid?

Before explaining, the tool builds a RegExp from your pattern and flags. If that throws — for example an unbalanced parenthesis, a dangling backslash, or an invalid quantifier — it returns the engine’s error message instead of a partial token list.

Regex Explainer

Deep dive guides

How regex explainer works

Frequently asked questions

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