Camelcased and dashed text to underscored text in Python

underscorize.py

import re

def underscorize(x):
    """
    Camelcased and dashed text to underscored text.
    Examples:
        >>> underscorize('CamelCasedText')
        'camel_cased_text'
        >>> underscorize('camel-cased-text')
        'camel_cased_text'
        >>> underscorize('Camel-Cased-Text')
        'camel_cased_text'
    """
    return re.sub(r'([^_])([A-Z])', r'\1_\2', re.sub(r'-', '_', x)).lower()