When searching for a whole word if that word is part of a previous word on a line it won't be found, if the regular expression option is on.
name form me test
blankform form thru me
Search for "ME", Match whole words only, regular expressions on, only the second occurance will be found
Search for "FORM", Match whole words only, regular expressions on, only the first occurance will be found.
When Use regular expressions is off, it will find all occurances.