Regex Tester
Experiment with patterns and flags against sample strings. See matches with indexes and try a replacement string.
Flags
Uses String.replace. With flag g, all matches are replaced.
FAQ
Flavors matter?
This uses JavaScript `RegExp`—PCRE lookahead or backreference rules differ from grep or Ruby; confirm engine when porting patterns.
Catastrophic backtracking?
Nested quantifiers like `(a+)+$` can freeze the tab; keep alternation tight and test with short strings first.
Unicode property escapes?
Require `u` flag and ES2018+ support—Safari versions vary; verify target matrix.
Replacement callbacks?
We only show simple string replacement—`$1` groups work but function replacers are out of scope here.
Security?
Never run user-supplied regex on servers without sandboxing—ReDoS is real.
Performance on huge haystacks?
Megabyte logs belong in ripgrep, not interactive browsers.
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