Python upper to lower with underline

upper2lower.py

def upper2lower(string):
    string1 = ''
    start = 0
    for m in re.finditer('[A-Z]', string):
        if m.start() != 0:
            string1 += string[start+1:m.start()] + '_' + m.group(0).lower()
        else:
            string1 += string[start:m.end()].lower()
        start = m.start()
    string1 += string[start+1:]
    return string1