def is_mobile(user_agent):
    """ Checks if the user browser from the given user agent is mobile.

    Args:
        user_agent: A given user agent.

    Returns: True if the browser from the user agent is mobile.

    """
    if user_agent:
        b = reg_b.search(user_agent)
        v = reg_v.search(user_agent[0:4])
        return b or v
    return False