How to use python regex to replace using captured group?

Question

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'

Asked By: Eric Wilson

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Source

Answer 1

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)

Answered By: mac

Answer 2

Try this:

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

Answered By: CAB

Answer 3

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)

Answered By: justcompile

Answer 4

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.

Answered By: ThorSummoner

How to use python regex to replace using captured group?

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