<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_level(tag):
 """ Match the header level in the given tag name, or None if it's not a header tag. """

        m = re.match(r'^h([123456])$', tag, flags=re.IGNORECASE)
        if not m:
            return None
        return int(m.group(1))