re-builder
re-builder is an interactive autoloaded compiled Lisp function in `re-builder.el
'.
(re-builder)
Construct a regexp interactively.
This command makes the current buffer the "target" buffer of
the regexp builder. It displays a buffer named "*RE-Builder*"
in another window, initially containing an empty regexp.
As you edit the regexp in the "*RE-Builder*" buffer, the
matching parts of the target buffer will be highlighted.