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

Solution 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)

Solution 2:

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)

Solution 3:

Off topic, For numbered capture groups:

#/usr/bin/env pythonimport 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.

Solution 4:

Try this:

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