def clean_semver(cls, version_spec):
        return re.sub(r"([<>=~])\s+?v?", "\\1", version_spec, re.IGNORECASE)
def test_regex_bad_input():
    cases = (
        'abc',
        '123abc',
        'abc123',
        '',
    )

    num_only = inputs.regex(r'^[0-9]+$')

    for value in cases:
        yield assert_raises, ValueError, lambda: num_only(value)