Paste text, enter what to find and what to put in its place, and see the result immediately.
Plain mode treats everything you type as literal text, so characters like a dot or a bracket need no escaping. Whole word mode only matches complete words, so replacing cat will not touch category.
In regular expression mode the Find box is a pattern and the replacement can use $1, $2 and so on to refer to captured groups, and $& for the whole match. The status line reports how many replacements happened so that a bad pattern is easy to spot.
Enable regular expressions and search for \n. Use the replacement box for what should go in its place. To remove line breaks entirely, the Remove Line Breaks tool offers more control.
Check the match case and whole word options. If regular expressions are on, a special character in your text, such as a parenthesis, may have been read as part of the pattern.
Yes, in regular expression mode. For example, finding (\w+) (\w+) and replacing with $2 $1 swaps each pair of words.