有没有办法在Python的re.sub（）中的替换字符串中使用正则表达式？

Question

In Python in the re module there is the following function:

re.sub(pattern, repl, string, count=0, flags=0) – Return the string obtained by replacing the leftmost non-overlapping occurrences of pattern in string by the replacement repl. If the pattern isn't found, string is returned unchanged.

I've found it can work like this:

print re.sub('[a-z]*\d+','lion','zebra432') # prints 'lion'

I was wondering, is there an easy way to use regular expressions in the replacement string, so that the replacement string contains part of the original regular expression/original string? Specifically, can I do something like this (which doesn't work)?

print re.sub('[a-z]*\d+', 'lion\d+', 'zebra432')

I want that to print 'lion432' . Obviously, it does not. Rather, it prints 'lion\\d+' . Is there an easy way to use parts of the matching regular expression in the replacement string?

By the way, this is NOT a special case. Please do NOT assume that the number will always come at the end, the words will always come in the beginning, etc. I want to know a solution to all regexes in general.

Thanks

Answer 1

Place \\d+ in a capture group (...) and then use \\1 to refer to it:

>>> import re
>>> re.sub('[a-z]*(\d+)', r'lion\1', 'zebra432')
'lion432'
>>>
>>> # You can also refer to more than one capture group
>>> re.sub('([a-z]*)(\d+)', r'\1lion\2', 'zebra432')
'zebralion432'
>>>

From the docs :

Backreferences, such as \\6 , are replaced with the substring matched by group 6 in the pattern.

Note that you will also need to use a raw-string so that \\1 is not treated as an escape sequence.