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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_identifier(string):
 """Check if string could be a valid python identifier :param string: string to be tested :returns: True if string can be a python identifier, False otherwise :rtype: bool """

    matched = PYTHON_IDENTIFIER_RE.match(string)
    return bool(matched) and not keyword.iskeyword(string)