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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 _generate_mark_code(rule_name):
 """Generates a two digit string based on a provided string Args: rule_name (str):
 A configured rule name 'pytest_mark3'. Returns: str: A two digit code based on the provided string '03' """

    code = ''.join([i for i in str(rule_name) if i.isdigit()])
    code = code.zfill(2)
    return code