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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 validate_regexp(pattern, flags=0):
 """ Validate the field matches the given regular expression. Should work with anything that supports '==' operator. :param pattern: Regular expresion to match. String or regular expression instance. :param pattern: Flags for the regular expression. :raises: ``ValidationError('equal')`` """

    regex = re.compile(pattern, flags) if isinstance(pattern, str) else pattern

    def regexp_validator(field, data):
        if field.value is None:
            return
        if regex.match(str(field.value)) is None:
            raise ValidationError('regexp', pattern=pattern)
    return regexp_validator