Question 1

Is the engine the same as on other regex sites?

Accepted Answer

This page runs your pattern with the JavaScript regex engine built into your own browser (ECMAScript). Nothing is sent to a server. Behaviour can differ slightly between Chrome / Firefox / Safari (mostly how soon new syntax is supported), and server-side flavors (Python / Java / PCRE) are similar but not identical, so check the engine you will actually deploy to.

Question 2

Why does my pattern keep showing no matches?

Accepted Answer

Usual suspects: no g flag, so only the first hit counts; ^ and $ need m to work per line in multiline text; . does not match line breaks unless you turn on s; and backslashes sometimes get swallowed before reaching this page. The token explainer below shows how the engine actually reads your pattern.

Question 3

How do $1 and $<name> work in the replacement?

Accepted Answer

The replacement string follows JavaScript String.replace rules: $& is the whole match, $1 / $2 are the first / second capture groups, $<name> is the named group, and $$ is a literal $. So a literal $ that is not followed by a reference must be written as $$.

Question 4

What is the difference between greedy and lazy matching?

Accepted Answer

Quantifiers are greedy by default: .* eats as many characters as possible before the rest of the pattern matches. Adding ? makes them lazy (.*?), matching as few as possible. From " ", <.*> matches the whole string, while <.*?> matches only - try both here and watch the highlight.

Question 5

Will my sensitive text be uploaded?

Accepted Answer

No. Matching, highlighting and replacing all happen in your browser's memory - this is a purely static page with no backend. Your text and pattern are only saved in your own device's localStorage so you can resume them next visit; the server never sees them.

Question 6

Why does my pattern freeze the page?

Accepted Answer

That is catastrophic backtracking: with shapes like (a+)+ or ^(\w+\s?)*$ - a quantifier applied to a group that already contains a quantifier - the number of branches the engine must try explodes exponentially as soon as one character fails to match. 55 characters can take a minute and a half. This page runs matching on a separate thread with a hard 1 second timeout, so a timeout aborts that run and tells you why instead of freezing the tab. There is usually a nesting-free equivalent, e.g. ^\w+(?:\s\w+)*$ instead of ^(\w+\s?)*$.

Question 7

How do I match line breaks? Why does . skip them?

Accepted Answer

The dot matches any character except a line break. Two options: ① turn on the s (dotAll) flag so . also matches line breaks; ② leave s off and write [\s\S] (or the equivalent [^]), which explicitly means "any character, line breaks included". If instead you want ^ and $ to work per line, turn on m.

Question 8

Why does \d miss full-width digits? How do I match Chinese text?

Accepted Answer

\d is the same as [0-9] and only accepts half-width ASCII digits; full-width digits belong to the Unicode Nd category. For digits in any script turn on u and use \p{Nd}; for Chinese use [\u4e00-\u9fa5] or, with u, \p{Script=Han}. The cheatsheet's "Unicode & advanced" section here has one-click entries for both.

Question 9

What happens if the replacement references a group that does not exist?

Accepted Answer

This one is easy to misread: $<name> becomes an empty string when the pattern has named groups but none called name, yet is kept literally when the pattern has no named groups at all; out-of-range numbers like $99 are kept literally too. The same string therefore behaves differently across patterns, so this page shows an inline note under the replacement box.

Question 10

Why do (?i) or (?>a+) fail with a syntax error?

Accepted Answer

JavaScript supports neither inline flags like (?i) nor Python/PCRE-only syntax such as (?>…), \A, \Z, [[:alpha:]] or \R - they are syntax errors here. When such syntax is detected this page adds a note that it is backend-flavour regex; test those patterns in the engine you actually target, since this page only toggles the six flag buttons in the UI.

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