What regex flavor does the explainer describe?

It describes JavaScript (ECMAScript) regular expressions, the same flavor used by browsers and Node.js. Most syntax - character classes, quantifiers, groups, and anchors - is shared with PCRE, Python, and Java, but some advanced features differ. If your pattern is meant for another language, treat the shared constructs as accurate and double-check flavor-specific extensions.

What is the difference between a greedy and a lazy quantifier?

A greedy quantifier like + or * matches as much text as possible before backtracking, while a lazy quantifier - the same symbol followed by ? such as +? or *? - matches as little as possible. The explainer labels lazy quantifiers explicitly so you can see when a pattern is set to stop at the first opportunity rather than the last.

What does a named capturing group like (? \d{4}) mean?

It captures the matched text and stores it under the name "year" as well as by number, so you can reference it later as groups.year instead of a numeric index. The explainer identifies named groups by their name and describes the pattern inside them, making it clear what each captured value represents.

Can the explainer handle lookahead and lookbehind?

Yes. Positive and negative lookahead (?=...) and (?!...), and positive and negative lookbehind (?<=...) and (?<!...), are each labelled with what they assert and are indented like other groups. Lookarounds check that text does or does not appear at a position without including it in the match, which the descriptions make explicit.

Is it safe to paste a regex that matches private data?

Yes. The pattern is analysed entirely in JavaScript inside your browser - it is never sent to a server, never logged, and the tool works with your connection disabled once loaded. You can safely explain patterns from private codebases or ones that match personal or proprietary formats.

Why does the explainer say my pattern is invalid?

The pattern is compiled with the real RegExp engine first, so an unbalanced bracket or parenthesis, an invalid escape, or an unknown flag is reported with the engine's exact message. Fix the reported issue - most often a missing closing ) or ] - and the breakdown will appear.

How is the explainer different from a regex tester?

A regex tester runs a pattern against sample text and shows what it matches, while the explainer describes what the pattern means without needing any test input. Use the explainer to understand or document an unfamiliar pattern, then use the Regex Tester on this site to confirm it behaves as expected against real data.

Do I need an account?

No account, no email, no trial. The regex explainer runs entirely in the tab, which is also why there is no usage cap to buy your way past: nothing is being metered on our side.

Is there a size limit?

Not one we impose. Because nothing is uploaded, the limit is whatever your browser can hold at once - comfortably a few megabytes, noticeably slower in the tens, and unreliable past a hundred or so, where a command-line tool belongs.

Regex Explainer

Q: Is it safe to paste a regex that matches private data?

Yes. The pattern is analysed entirely in JavaScript inside your browser - it is never sent to a server, never logged, and the tool works with your connection disabled once loaded. You can safely explain patterns from private codebases or ones that match personal or proprietary formats.

Q: Why does the explainer say my pattern is invalid?

The pattern is compiled with the real RegExp engine first, so an unbalanced bracket or parenthesis, an invalid escape, or an unknown flag is reported with the engine's exact message. Fix the reported issue - most often a missing closing ) or ] - and the breakdown will appear.

Q: How is the explainer different from a regex tester?

A regex tester runs a pattern against sample text and shows what it matches, while the explainer describes what the pattern means without needing any test input. Use the explainer to understand or document an unfamiliar pattern, then use the Regex Tester on this site to confirm it behaves as expected against real data.

Q: Do I need an account?

No account, no email, no trial. The regex explainer runs entirely in the tab, which is also why there is no usage cap to buy your way past: nothing is being metered on our side.

Q: Is there a size limit?

Not one we impose. Because nothing is uploaded, the limit is whatever your browser can hold at once - comfortably a few megabytes, noticeably slower in the tens, and unreliable past a hundred or so, where a command-line tool belongs.

Paste a regular expression and get a clear, token-by-token explanation in plain English, including flags, groups, and quantifiers.

Regular Expression

//gi

Flags:

This pattern has 2 capturing groups with 2 flags (gi) and is broken down into 14 parts below.

Flags

gglobal - find all matches in the input, not just the first one

iignore case - letters match regardless of upper or lower case

Breakdown

^Start-of-string anchor (or start of a line with the m flag)

(?<user>Start of a named capturing group "user" - saves its match under that name

[a-z0-9._%+-]A single character from the set: the range "a" to "z", the range "0" to "9", the character ".", the character "_", the character "%", the character "+", the character "-"

+Repeated one or more times

)End of the group

@A literal "@" character

(?<domain>Start of a named capturing group "domain" - saves its match under that name

[a-z0-9.-]A single character from the set: the range "a" to "z", the range "0" to "9", the character ".", the character "-"

+Repeated one or more times

\.A literal "." character (the backslash removes its special meaning)

[a-z]A single character from the set: the range "a" to "z"

{2,}Repeated 2 or more times

)End of the group

$End-of-string anchor (or end of a line with the m flag)

What is a Regex Explainer?

A Regex Explainer takes a regular expression and translates it into plain English, one construct at a time. Instead of squinting at a dense line like ^(?<user>[a-z0-9._%+-]+)@... you get a labelled breakdown: this part is an anchor, this is a named capturing group, this character class means "a lowercase letter, a digit, a dot, or a hyphen", and this quantifier means "one or more times". It turns a wall of symbols into a checklist you can read.

Regular expressions are compact by design, which makes them powerful but hard to review. The same pattern can be written many ways, and a single stray backslash or a greedy quantifier changes the meaning completely. Reading an unfamiliar regex - in a code review, an old config file, or a Stack Overflow answer - usually means mentally simulating the engine. This tool does that simulation for you and shows the result as an ordered, indented list that mirrors the structure of the pattern, including nested groups and lookarounds.

Everything runs client-side on the same JavaScript RegExp engine your code uses, so the flag behaviour and validity you see here match production. Your pattern is never uploaded, nothing is logged, and the tool keeps working offline once the page has loaded. That matters when the regex you are trying to understand comes from a private codebase or matches sensitive data formats.

The explainer tells you what a pattern says, not whether it is the right pattern for your data - to check that, run it against real input in the Regex Tester. Because it compiles with the browser's own engine, constructs that exist in PCRE but not in ECMAScript behave in two different ways. Possessive quantifiers such as a++, atomic groups (?>...) and recursion (?R) are rejected outright, with the engine's own message. The subtler case is \A and \z: without the u flag JavaScript treats them as identity escapes, so a pattern copied from PHP or Python is explained as matching a literal A or z rather than a string boundary, and no error is raised. Nested quantifiers like (a+)+ are described normally too; catastrophic backtracking is a runtime property of the input, not something visible in the structure.

How to use the Regex Explainer?

Paste your regular expression
Enter the pattern without the surrounding slashes - just the expression itself. Click Load Sample to start from a worked email-matching example.
Set the flags
Toggle the flags the pattern uses (g, i, m, s, u, y, d). The explainer reports what each flag does and validates the pattern against that flag combination.
Read the breakdown
Each token is listed in order with a plain-English description. Nested groups are indented so the structure of the pattern is visible at a glance.
Copy the explanation
Copy the full plain-text breakdown to paste into a code comment, a pull-request note, or documentation so the next reader does not have to decode it.

Key Features

Token-by-Token Breakdown
Every anchor, character class, quantifier, group, and escape gets its own line with a description, in the order the engine reads them
Group and Lookaround Aware
Capturing, non-capturing, named groups, and all four lookaround types are labelled and indented so nested structure is easy to follow
Flag Explanations
The g, i, m, s, u, y, and d flags are each described in context, so you understand how they change the whole match, not just the syntax
Real Engine Validation
Patterns are validated with the browser native RegExp engine, so an invalid expression is caught with the exact error before you rely on it

Frequently Asked Questions

Paste the pattern into the explainer and read the token-by-token breakdown, which describes each construct in plain English in the order the engine applies it. Nested groups are indented so you can see the structure. This turns mentally simulating the engine into simply reading a labelled list.

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Learn more

Read the full Regex Explainer guide

Command Palette

Regex Explainer

What is a Regex Explainer?

How to use the Regex Explainer?

Paste your regular expression

Set the flags

Read the breakdown

Copy the explanation

Key Features

Token-by-Token Breakdown

Group and Lookaround Aware

Flag Explanations

Real Engine Validation

Frequently Asked Questions

Related Tools

Email Extractor

Regex Replace

Regex Tester

Regex Builder

Comments

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