Plain text or regular expression
In plain mode the tool looks for exactly what you type. A dot is just a dot and an asterisk is just an asterisk, so you can paste a file name, a price or a snippet of code into the Find box without escaping anything.
Regex mode treats the Find box as a JavaScript regular expression, which is what lets you change a whole pattern at once. The replacement can reuse parts of the match:
$1,$2and so on insert the text of a group.$<name>does the same for a named group.$&inserts the whole match, and$$inserts a dollar sign.\nand\tinsert a line break and a tab.
For example, find (\d{4})-(\d{2})-(\d{2}) and replace with $3/$2/$1 to turn 2024-05-12 into 12/05/2024. In regex mode ^ and $ match at the start and end of every line, so find ^ and replace with > to quote each line. Windows (CRLF) and old Mac line breaks are read as plain line breaks first, so a loaded Windows file behaves the same way, and the result has plain (LF) line breaks.
Using several rules in order
Rules run from the top, and each one sees the text the previous rule produced. That makes cleanups easy to build: one rule to remove double spaces, one to fix a spelling, one to change a date format.
Order matters. If rule 1 turns a into b and rule 2 turns b into c, every a ends up as c. Use the arrows on a rule to move it, the checkbox to switch it off without deleting it, and the match count on each rule to see what it did.
A rule with an empty Find box is ignored. A rule whose regular expression is not valid is skipped, and the reason is shown on the rule, while the others still run.
Whole words, capital letters and step by step review
Whole word stops cat from matching inside concat or cats. A word here is a run of letters, accents, digits and underscores in any script, so it works for accented and non-Latin text. A hyphen is not part of a word, so cat also matches inside cat-like.
Match case is off by default, so find hello also finds Hello and HELLO. The replacement is inserted exactly as you typed it, so the capital letters of the original are not copied across.
Step through is for changes you do not trust to a blanket replace. Each match is shown with the text around it, and you choose Replace or Skip. Undo takes back the last choice, and the result is built from your choices without touching the text box until you choose Use as new text.
Why a regular expression can freeze a page, and the guard here
Some patterns take twice as long with every extra character. A nested repeat such as (a+)+$ on a long run of a letters followed by b is the classic case. This is called catastrophic backtracking, and in an ordinary page it freezes the browser tab until it finishes, which can be minutes or longer.
This tool runs every search in a separate worker. If one takes more than 3 seconds, the worker is stopped, the page stays responsive, and you see a message instead of a frozen tab. Plain rules are escaped before they are run, so they cannot trigger the problem.