Turning regexp-opt back to a list of words

[rgb]

Lets say some mode writer used regexp-opt to create their font-lock-
keyword expression.
So only something like this string is in the mode source, not the
original word list.

"\\<\\(d\\(?:o\\|efault\\)\\|else\\(if\\)?\\|f\\(?:oreach\\|unction\\)\
\|if\\|switch\\|t\\(?:hrow\\|rap\\)\\|w\\(?:here\\|hile\\)\\)\\>"

The mode is not supported and I want to take over maintenance.
That means adding words.
Is there any way to turn this back into a list so the new words can be
added?