[boost] Regex-User: ISBN validation

Hi, we need to validate ISBN. With another REGEX-implementation we uses (?=.{13})\d{1,5}( |\-)\d{1,7}\1\d{1,6}\1(\d|X). But this seem not to work. Only the shorter version "\d{1,5}( |\\-)?\\d{1,7}\\1\\d{1,6}\\1(\d|X)" works, but how to check the 13 digits? Thanx in advanced Detlef