improve Spelling rules by using word boundaries

By using regex word boundary (\b) delimiters, the spelling rule
applies to individual words rather than a word that might contain the
regex filter. For example, "\b[cC]he\b" will match only "che" and "Che"
rather than a regex filter without word boundary delimiters, for
example, "[cC]he" that would match misspelled words that contain the
regex, such as "aache" or "chemitsry".