Question 1

Which regular expression syntax does this tester use?

Accepted Answer

This tester uses the JavaScript regular expression engine built into modern browsers. It supports character classes, quantifiers, lookarounds, named capture groups and supported ECMAScript flags.

Question 2

What do the regex flags mean?

Accepted Answer

g finds every match, i ignores case, m makes ^ and $ work at line boundaries, s lets dot match newlines, u enables Unicode mode, and y uses sticky matching from the current position.

Question 3

How do I use capture groups in a replacement?

Accepted Answer

Wrap a part of your pattern in parentheses, then reference it with $1, $2 and so on in the replacement field. Named groups can be referenced with $<name>. The preview uses JavaScript replacement rules.

Question 4

Does this regex tester upload my text?

Accepted Answer

No. Matching and replacement run entirely in your browser. Patterns and test text are not sent to a server.