How to use python regex to replace using captured group? [duplicate]

This question already has answers here: Handling backreferences to capturing groups in re.sub replacement pattern (2 answers) Closed 3 years ago.

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 '开发者_StackOverflowthe 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'

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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)

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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.

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Try this:

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