Set face of each match of phrase REGEXP to FACE.
If called interactively, replaces whitespace in REGEXP with
arbitrary whitespace and makes initial lower-case letters case-insensitive.
If Font Lock mode is enabled in the buffer, it is used to
highlight REGEXP. If Font Lock mode is disabled, overlays are
used for highlighting; in this case, the highlighting will not be
updated as you type. (fn REGEXP &optional FACE)