import re

splitters = ("_", "-")

s = "12_xxxxxxx_xxxxxxx_xxxx_xxxxxx_xx.doc"

l = [""]
for part in re.split("(%s)" % "|".join(re.escape(splitter) for splitter in splitters), s):
	if len(l[-1]) < 14:
		l[-1] += part
	elif part not in splitters:
		l.append(part)
print l