Mastering Python Regular Expressions

Backreferences

As we've mentioned previously, one of the most powerful functionalities that grouping gives us is the possibility of using the captured group inside the regex or other operations. That's exactly what backreferences provide. Probably the best known example to bring some clarity is the regex to find duplicated words, as shown in the following code:

>>>pattern = re.compile(r"(\w+) \1")
>>>match = pattern.search(r"hello hello world")
>>>match.groups()
('hello',)

Here, we're capturing a group made up of one or more alphanumeric characters, after which the pattern tries to match a whitespace, and finally we have the \1 backreference. You can see it highlighted in the code, meaning that it must exactly match the same thing it matched as ...