文件读写问题

import re
f = open('F:\\a.txt','r')
f2 = open('F:\\aa.txt','w')
a = f.readlines()
for i in a:
b = re.search(r'^.*[0-9]\s+(.+)',i)
print b.group(1)
f2.write(b.group(1))
f.close()
f2.close()
为什么会出错，AttributeError: 'NoneType' object has no attribute 'group'
求解