How to use python regex to replace using captured group?

Suppose I want to change the blue dog and blue cat wore blue hats to the gray dog and gray cat wore blue hats.

With sed I could accomplish this as follows:

$ echo 'the blue dog and blue cat wore blue hats' | sed 's/blue \(dog\|cat\)/gray \1/g'

How can I do a similar replacement in Python? I've tried:

>>> import re
>>> s = "the blue dog and blue cat wore blue hats"
>>> p = re.compile(r"blue (dog|cat)")
>>> p.sub('gray \1',s)
'the gray \x01 and gray \x01 wore blue hats'

You need to escape your backslash:

p.sub('gray \\1', s)

alternatively you can use a raw string as you already did for the regex:

p.sub(r'gray \1', s)

As I was looking for a similar answer; but wanting using named groups within the replace, I thought I'd add the code for others:

p = re.compile(r'blue (?P<animal>dog|cat)')
p.sub(r'gray \g<animal>',s)

Off topic, For numbered capture groups:

#/usr/bin/env python
import re

re.sub(
    pattern=r'(\d)(\w+)', 
    repl='word: \\2, digit: \\1', 
    string='1asdf'
)

> word: asdf, digit: 1

Python uses literal backslash, plus one-based-index to do numbered capture group replacements, as shown in this example. So \1, entered as '\\1', references the first capture group (\d), and \2 the second captured group.

Try this:

p.sub('gray \g<1>',s)