How this tester works

Patterns run with your browser’s own JavaScript regex engine, so what matches here matches in Node.js and front-end code. Each run happens in a background worker with a time limit. A pattern that backtracks catastrophically is stopped, and the page doesn’t freeze.

Flags

g finds every match instead of stopping at the first.
i ignores case.
m makes ^ and $ match at the start and end of each line, not only the whole text.
s lets . match newlines too.
u treats the pattern as Unicode, so \u{1F600} and \p{L} (any letter) work.

Cheat sheet

.        any character except newline
\d \w \s digit, word character [A-Za-z0-9_], whitespace
\D \W \S the opposite of each
[abc]    one of a, b, c         [^abc]  anything except those
^ $      start / end            \b      word boundary
*  +  ?  0+, 1+, 0 or 1         {2,5}   between 2 and 5
*? +?    lazy: as few as possible
(x)      capture group          (?:x)   group without capturing
(?<n>x)  named group            \1      back-reference to group 1
x(?=y)   x followed by y        x(?!y)  x not followed by y
(?<=y)x  x preceded by y        (?<!y)x x not preceded by y

Replacement patterns

In the replacement, $1 inserts the first capture group, $<name> a named group, $& the whole match and $$ a literal dollar sign. For example, pattern (\d{4})-(\d{2})-(\d{2}) with replacement $3/$2/$1 turns 2024-05-31 into 31/05/2024.

Catastrophic backtracking

Nested quantifiers like (a+)+$ or (\w+\s?)*$ can take exponential time on input that almost matches. On a server, this is a denial-of-service risk called ReDoS. Avoid quantifying a group that already contains a quantifier, and make alternatives mutually exclusive.

Differences from other languages

Java and Python use mostly the same syntax, with a few differences. Python writes named groups as (?P<name>…). Java strings need every backslash doubled ("\\d+"). Neither has a g flag: use Matcher.find() in a loop, or re.findall.

Regex tester

Matches