def is_keyword(v):
    """ Check if a value is of the format required to be a call to a header keyword

    Parameters
    ----------
    v : str
      Value to be tested

    Returns
    -------
    valid : bool
      True if 'v' has the correct format to be a header keyword, False otherwise.
    """
    valid = True
    if ("," in v) or ("." not in v):
        # Either an array or doesn't have the header keyword format (i.e. a fullstop)
        return False
    # Test if the first element is an integer
    vspl = v.split(".")
    try:
        int(vspl[0])
    except ValueError:
        valid = False
    # Test if there are two parts to the expression
    if len(vspl) != 2:
        return False
    # Test if the second element is a string
    try:
        if valid is True:
            int(vspl[1])
            # Input value must be a floating point number
            valid = False
    except ValueError:
        # Input value must be a string
        valid = True
    return valid