Question 1

Which regex flavor does this use?

Accepted Answer

JavaScript's built-in regex engine (the one behind RegExp and String.prototype.match/replace), since everything here runs client-side in your browser rather than on a server. It's very close to PCRE but has a few differences - for example, lookbehind support depends on the engine version, and named groups use (?<name>...) syntax.

Question 2

Why does my pattern only match once?

Accepted Answer

By default, without the g (global) flag, a regex stops after its first match - this mirrors how RegExp.exec() and String.match() actually behave in JavaScript. Enable the g flag above to find every match in the test string instead of just the first one.

Question 3

What's the difference between (...) and (?:...)?

Accepted Answer

Both group parts of a pattern together, but only (...) - a capturing group - saves what it matched so you can reference it later (as $1, $2, ... in a replacement, or in the match's group list). (?:...) is a non-capturing group: it groups and applies quantifiers without keeping the match around, which is useful when you need grouping but don't care about the captured text.

Question 4

How do named groups and $<name> replacements work?

Accepted Answer

Write (?...) instead of (...) to capture a group under a name instead of just a number. In Replace mode, refer to it in the replacement text as $ - for example, (?\d{4})-(?\d{2}) replaced with $/$ rearranges a date without needing to remember group numbers.

Question 5

What's the difference between a greedy and a lazy quantifier, like + versus +??

Accepted Answer

A greedy quantifier (+, *, {2,4}) matches as much as it possibly can, then backs off only if needed to let the rest of the pattern match. A lazy quantifier - the same symbol followed by ? - matches as little as possible instead, expanding one character at a time only when required. For example, <.+> against greedily matches the whole , while <.+?> lazily matches just .

Question 6

What do lookahead and lookbehind actually check, if they don't consume any characters?

Accepted Answer

They test whether a pattern appears immediately before or after the current position without including it in the match itself - so (?=...) and (?<=...) check ahead/behind for something that must be there, while (?!...) and (?<!...) check that it must not be there. That's why you can use \d+(?=px) to match a number only when it's followed by 'px', while 'px' itself never shows up in the matched text.

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