Regex Find and Replace Online

The pattern runs on the RegExp engine of your browser and the replacement follows the rules of String.prototype.replace(): $1…$99 insert a numbered group, $<name> a named group, $& the whole match, $` and $' the text before and after the match, and $$ a dollar sign. A reference to a group that does not exist is left as written. With the g flag every match is replaced, without it only the first; m makes ^ and $ match at each line, s lets . match line breaks, i ignores case, u enables \p{…} and y matches only at the start position.

JavaScript itself does not read backslashes in a replacement. With Escapes in replacement the tool adds them, as text editors do: \n, \t, \r, \\, \xHH and \u{…} insert a character, \U and \L turn what follows into upper or lower case until \E, and \u and \l change only the next character, so _([a-z]) → \u$1 converts snake_case to snakeCase. Plain text searches for the characters as typed and takes the replacement literally; Whole word skips matches that are part of a longer word.

The search runs in a Web Worker with a limit of 1.5 seconds and 20,000 matches: a pattern with catastrophic backtracking such as (a+)+$ is stopped with a message and the page stays responsive. Matches of length zero (^, \b, x*) are handled like in JavaScript, one per position. The two previews mark the matches in the input and the inserted text in the output. The text never leaves your browser.