Hmm, that does 'work'. But what definition of regex are you using? In perlre, it says '\b' should match a word boundary transition, whether its from a word to a non-word or a non-word to a word character transition. So, '\b' should match "abc def ghi" (among other places) both before the "d" and after the "f". '\B' would match the places in between the letters, like between "a" and "b".

Update:Ahh, ok. it all makes sense now :-)