mongoFilterToDoc (RegExpFilter fn (reg, opts)) = [ fieldName fn  DB.:= DB.RegEx (DB.Regex reg opts)]
mongoFilterToDoc (RegExpFilter fn (reg, opts)) = [ fieldName fn  DB.:= DB.RegEx (DB.Regex reg opts)]
mongoFilterToDoc (RegExpFilter fn (reg, opts)) = [ fieldName fn  DB.:= DB.RegEx (DB.Regex reg opts)]