import re

rx = r'([0-9]+)|([A-Z]+)|[a-z]+'
w = "XXXxxx999XX"

def repl(m):
	if m.group(1):
		return '9'
	elif m.group(2):
		return 'X'
	else:
		return 'x'

print(re.sub(rx, repl, w))