JavaScript Regex Tester

Separate matches, captures and flags

A match is the text found by the whole pattern; captures are values taken by its parentheses. An optional group that did not participate is shown as null. namedGroups lists named captures separately.

The default g flag finds multiple matches. Without g, only the first is returned. The i/m/s/u flags change case, line boundaries, dot/newline and Unicode handling. Do not assume that another regex engine will produce identical results.

Check offsets with a small example

Example fills the pattern below, its text and the gi flags. The matches are cat 42 and CAT 7, with captures cat/42 and CAT/7. The first spans index 0 to end 6; the second follows the newline, spanning index 7 to end 12.

index/end are UTF-16 offsets with an exclusive end. For example, . with gu against 😀 returns one match spanning [0,2). Empty-string matches are retained, so an invisible match is different from no match.

Pattern: ([a-z]+)\s+(\d+)
Flags: gi
Text:
cat 42
CAT 7

Read time and output limits correctly

Testing runs in a separate Worker and is stopped after one second including startup. Timeout, syntax error and cancellation are not no-match results and do not count as successful runs. Editing or clearing the input also invalidates old Worker results.

Only a prefix is returned when 200 matches or 50,000 code units of match/capture values would be exceeded. If the first match has very large captures, the displayed count can even be zero. Read the limit warning and reduce the input or captures.

A completed test does not certify your application’s validation or security. Use JSON Validator for JSON grammar rather than substituting a regex, or Random String Generator when you need sample strings to test.