Return a regexp which matches words, ignoring punctuation.
Given STRING, a string of words separated by word delimiters,
compute a regexp that matches those exact words separated by
arbitrary punctuation. If LAX is non-nil, the end of the string
need not match a word boundary unless it ends in whitespace.
Used in `word-search-forward', `word-search-backward',
`word-search-forward-lax', `word-search-backward-lax'.