Paste your text, say what to look for and what to put in its place. By default the search is taken literally, so a full stop means a full stop rather than "any character" — that surprise is the usual reason a replace goes wrong. Tick the regular expression box when you actually want a pattern, and you will be told plainly if the expression does not compile. Everything happens in this page; the text is not uploaded.
This page loads no libraries at all — the work is done by the browser itself. Your file is read on your own machine and never sent anywhere — open your browser's network tab and watch while you use it.
Only if you ask for it. By default every character is taken literally, so searching for a.b finds a.b and leaves axb alone. That is the opposite of what most replace tools do, and it is the reason people end up with mangled text: a full stop, a plus sign or a bracket quietly turns into a wildcard. Tick the box and the same box becomes a real expression, with capture groups available as $1, $2 and so on.
You are told, in plain words, that it is not a valid regular expression, along with what the browser objected to. Nothing is replaced. A tool that swallows the error and returns your text unchanged is worse than one that complains, because you cannot tell the difference between "no matches" and "it never ran".
Not unless you tick the box. Left alone, Word and word are the same thing, which is what people usually want when tidying a draft. Tick it and the two are treated as different, which is what you want in code or in a list of identifiers.
No. The replacement runs in this page on your own machine, so a contract, a set of notes or anything else unpublished stays where it is. You can open your browser network tab and watch: nothing goes out.
Everything here is free and works the same way — in your browser.