Search backward from point for STRING, ignoring differences in punctuation.
Set point to the beginning of the occurrence found, and return point.
Unlike `word-search-backward', the end of STRING need not match a word
boundary, unless STRING ends in whitespace.
An optional second argument bounds the search; it is a buffer position.
The match found must not extend before that position.
Optional third argument, if t, means if fail just return nil (no error).
If not nil and not t, move to limit of search and return nil.
Optional fourth argument is repeat count--search for successive occurrences.
Relies on the function `word-search-regexp' to convert a sequence
of words in STRING to a regexp used to search words without regard
to punctuation.