An interactive Emacs tool to build a regexp (regular expression).
RE-Builder allows you to interactively develop a regexp on the contents of the currently active buffer. Immediately after you type each character, the subject buffer is updated to highlight all those strings that match the regexp you've created thus far. Only the first 200 matches (if there are more than 200) are shown.
A new window is created just above the minibuffer with a single a pair of quotation marks (""). This is the *RE-Builder* buffer.
You build your regexp between the quotation marks in this buffer.