check src_path complies with regex and generate new filename

def _file_path(self, src_path, dest, regex):
        """
        check src_path complies with regex and generate new filename
        """
        m = re.search(regex, src_path)
        if dest.endswith('/') or dest == '':
            dest += '{filename}'
        names = m.groupdict()
        if not names and m.groups():
            names = {'filename': m.groups()[-1]}
        for name, value in names.items():
            dest = dest.replace('{%s}' % name, value)
        # remove starting slash so path can't be absolute
        dest = dest.strip(' /')
        if not dest:
            progress_logger.error('destination path must not resolve to be null')
            raise GrablibError('bad path')
        new_path = self.download_root.joinpath(dest)
        new_path.relative_to(self.download_root)
        return new_path