Find and replace

Plain text means exactly that: every character you type is looked for as itself, so a dot is a dot and $1 is two characters. The page escapes the pattern before the engine sees it, which is the difference between this and a search box that quietly treats your full stop as "any character". Regular expression hands the pattern to the browser's own engine, so it is JavaScript's flavour: \d, \b, groups, lookahead and named groups all work, and in the replacement $1 is the first group, $& the whole match, $$ a literal dollar.

Whole words only wraps the pattern in word boundaries, which treats letters, digits and the underscore as word characters and everything else as a break — so it will not find "cat" inside "concatenate", and it will find "cat" in "cat-like". Ignore case is the engine's own case-insensitive matching, which handles accented letters as the browser does. First match only stops after one replacement instead of running through the document.

Nothing is changed in place: the result is a new text beside the original, and the original is still in the box above until you replace it yourself. A pattern that matches empty — a* against a string with no a — is refused rather than run, because the engine would insert the replacement between every character and the result is never what anyone meant. Input is capped at one million characters, and a regular expression that takes more than two seconds is stopped: some patterns backtrack exponentially, and a page that freezes is worse than one that says no.

Everything runs in this tab: nothing is uploaded and nothing you paste is stored. The Dark button keeps its own setting in this browser's local storage.