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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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