The following regex should match any document that IS NOT a pdf file: .*[.](?!pdf$).*$ This regex should match: Mynotes.doc It should fail on: Myinfo.pdf However, the above regex returns true on both using boost 1.33.0. But positive lookahead works fine: .*[.](?=pdf$).*$ Above regex correctly matches only documents ending in .pdf . Can anyone confirm this is a bug? Thanks, -Rob