python regular expression “\1”

Question

Can anyone tell me what does \"\\1\" mean in the following regular expression in Python?

re.sub(r\'(\\b[a-z]+) \\1\', r\'\\1\', \'cat in the the hat\')
         


        

Answer 1

From the python docs for the re module:

\number

Matches the contents of the group of the same number. Groups are numbered starting from 1. For example, (.+) \1 matches 'the the' or '55 55', but not 'thethe' (note the space after the group). This special sequence can only be used to match one of the first 99 groups. If the first digit of number is 0, or number is 3 octal digits long, it will not be interpreted as a group match, but as the character with octal value number. Inside the '[' and ']' of a character class, all numeric escapes are treated as characters.

Your example is basically the same as what is explained in the docs.