why the second print result is None:
import re
pattern=r"(.+) \1" #notice that there is whitespace here before the backslash
match=re.match(pattern,"abc abc")
if match:
print(1) #1 was printed
match=re.match(pattern,"abc abd")
print(match) # None
Additionally,please give me another example for the usage of '\2'. Thx a lot.