Regex Tester

Regular expressions are not one language. What works in Python, PCRE, Go or Java may not work here, and the differences bite.

JavaScript has no \A or \Z anchors — use ^ and $ with or without the m flag. It has no possessive quantifiers and no atomic groups. It has no inline flag syntax: (?i) is a syntax error, and the i goes in the flags box. Lookbehind ((?<=…)) is supported and has been since 2018, unlike in several other engines.

If you are writing a pattern for a JavaScript codebase, this is exactly right. If you are writing one for a Python script, test it in Python.

g — find every match rather than only the first. Always on here, because a tester that shows one match is not useful.

i — case-insensitive.

m — makes ^ and $ match at each line break instead of only at the ends of the whole string. This is the flag people forget when a pattern works on one line and fails on a file.

s — makes . match newlines too. Without it, . stops at every line break.

u — proper Unicode handling, needed for \p{…} property escapes and for matching astral characters like emoji as single units.

Some patterns take exponential time on inputs that do not match. Nested quantifiers are the classic cause — (a+)+b against a long run of as will hang a tab, and in production it is a denial-of-service vector known as ReDoS.

Matching here is capped at 5,000 results and you are told when the cap was hit, but a genuinely pathological pattern can still be slow because the cost is in the backtracking, not the match count. If typing a pattern makes the page freeze, that is the pattern telling you something important about what it would do to your server.

In the replacement box, $1 is the first capture group, $<name> is a named group, $& is the whole match and $$ is a literal dollar sign. The preview updates live, so you can see the result before running it anywhere real.