Read PNM header and initialize instance.

def _read_pnm_header(self, data):
        """Read PNM header and initialize instance."""
        bpm = data[1:2] in b"14"
        regroups = re.search(b"".join((
            b"(^(P[123456]|P7 332)\s+(?:#.*[\r\n])*",
            b"\s*(\d+)\s+(?:#.*[\r\n])*",
            b"\s*(\d+)\s+(?:#.*[\r\n])*" * (not bpm),
            b"\s*(\d+)\s(?:\s*#.*[\r\n]\s)*)")), data).groups() + (1, ) * bpm
        self.header = regroups[0]
        self.magicnum = regroups[1]
        self.width = int(regroups[2])
        self.height = int(regroups[3])
        self.maxval = int(regroups[4])
        self.depth = 3 if self.magicnum in b"P3P6P7 332" else 1
        self.tupltypes = [self._types[self.magicnum]]