import re


def parse_version_number(header):
    """Parses the version number from an accept header
    Args:
        header (str): Header content to parse for version number.

    Returns:
        Version number.
    
    """
    version_regex = '\w+/[VvNnDd]{3}\.\w+\.v([0-9\.]+)\+'
    match = re.search(version_regex, header)
    if match:
        found = match.group(1)
    else:
        found = None

    return found