Question 1

What regex flavour does this tester use?

Accepted Answer

It uses the browser's built-in JavaScript RegExp engine — exactly what you'd use in JavaScript, Node.js, TypeScript, or any browser-side code. It supports ES2018+ features like named groups, lookbehind assertions, and the s (dotAll) and d (indices) flags.

Question 2

How do I match across multiple lines?

Accepted Answer

Enable the m (multiline) flag so ^ and $ match the start/end of each line. Enable the s (dotAll) flag so . matches newline characters. For example, to match everything between tags: [\s\S]*? (or .*? with the s flag).

Question 3

What is the difference between greedy and lazy quantifiers?

Accepted Answer

Greedy quantifiers (*, +, {n,m}) match as much as possible. Lazy quantifiers (*?, +?, {n,m}?) match as little as possible. For example, <.+> on 'foo' greedily matches the whole string, while <.+?> matches only ''.

Question 4

How do I use capture groups in Replace mode?

Accepted Answer

In Replace mode, reference capture groups in the replacement string with $1, $2, etc. for numbered groups, or $<name> for named groups. For example, pattern (\w+)\s(\w+) with replacement $2 $1 will swap two words.

Question 5

Why does my regex match nothing with the u flag?

Accepted Answer

Unicode mode is stricter about syntax. Some patterns that work without u may throw an error with it (e.g. unescaped special chars in character classes). Also use \u{XXXX} notation for Unicode code points above U+FFFF when using the u flag.

Question 6

What does the d (indices) flag do?

Accepted Answer

Added in ES2022, the d flag adds a .indices property to each match containing the [start, end] position of the full match and each capture group. This tester already shows match indices, but the d flag also exposes group-level positions.

Question 7

How do I test a regex for an exact full-string match?

Accepted Answer

Anchor the pattern with ^ at the start and $ at the end: ^your-pattern$. This ensures the entire string must match, not just a substring. For multiline strings, also add the m flag if needed, or use \A and \Z alternatives.

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