Question 1

What regex flavour does this tester use?

Accepted Answer

The JavaScript (ECMAScript) flavour, run by your own browser. Patterns that work here work in JavaScript and TypeScript. Other languages such as Python, PCRE, Java or .NET differ in places, for example in lookbehind rules, named group syntax, possessive quantifiers and inline flags, so a pattern may need small changes before you use it elsewhere.

Question 2

What do the flags g, i, m, s, u and y do?

Accepted Answer

g finds every match instead of only the first. i ignores case. m makes ^ and $ match at each line. s lets the dot match line breaks. u turns on full Unicode matching, including code points above U+FFFF and \p{...} properties. y (sticky) only matches at the exact position where the search starts.

Question 3

Why does it say the pattern took too long?

Accepted Answer

Some patterns, such as (a+)+$ tested against a long run of a's followed by another character, backtrack an enormous number of times. The tester runs your pattern in a background worker and stops it after two seconds, so the page keeps working. Rewrite the pattern to avoid nested quantifiers over the same characters.

Question 4

How do I use capture groups in the replacement?

Accepted Answer

Write $1, $2 and so on for numbered groups, $<name> for named groups, $& for the whole match and $$ for a literal dollar sign. The preview shows the result exactly as String.prototype.replace would produce it.

Question 5

Is my text sent to a server?

Accepted Answer

No. The pattern and the test string stay in your browser. Only the pattern, flags and replacement are kept in the page address so you can bookmark or share a test, and the test string is never added to it.

Regex Tester: test JavaScript regular expressions live