How it works
Type in Find and the page counts the matches as you type. Replace all changes every one of them at once and says how many it replaced; Undo puts them back. Capitals are ignored unless you turn on Match case, and leaving Replace with empty deletes what you found. With Pattern off, every character means itself: a dot is a dot and $5 is five dollars.
Find and replace with a pattern
Turn on Pattern to search with a regular expression: \d+ finds any number, ^\s+ finds spaces at the start of a line, \n is a line break. Brackets in the pattern capture text that can be put back with $1, $2 in the replacement: find (\d+) and replace with [$1] to put every number in brackets.
Questions
Can I replace one match at a time?
No. It replaces every match at once. Make the search more specific, or use a pattern, to leave some alone.
Which regular expressions does it take?
JavaScript's, the same kind most code editors use. A pattern that is not valid is reported under the button, and nothing is changed.
Is my text sent anywhere?
No. Every change happens in this page on your device, and nothing is saved once you close it.