conditional replace of a string in python

Question

I want to perform conditional replace of string. if my

string = "abaabaaabaaaabaaabaaaaaabaababaaabaaaabaaaaaabaaaaabaaabaabaababaaabaaaabaaaabaabaaab"

and I want to replace a with 2 different character, so I want to change single "a" with "c" (a = c) and double "aa" will be with "d" "aa = d", replacement criteria is if odd number of "a" then first with single "a" will replace with "c", then every 2 "a" with "d", for example "ab = cb", "aab = db", "aaab = cdb" (first a = c and next 2 aa with d) aaaab = ddb. after replacement my string will be

"cbdbcdbddbcdbdddbdbcbcdbddbdddbcddbcdbdbdbcbcdbddbddbdbcdb"

Can someone guide me how to write code what I can use "if else" or regex

Answer 1

Here is a regex based solution that avoids 2 times reversing:

import re
string = "abaabaaabaaaabaaabaaaaaabaababaaabaaaabaaaaaabaaaaabaaabaabaababaaabaaaabaaaabaabaaab"

print (re.sub(r'(?<!a)a(?=(?:aa)*(?!a))', 'c', string).replace('aa', 'd'))

Output:

cbdbcdbddbcdbdddbdbcbcdbddbdddbcddbcdbdbdbcbcdbddbddbdbcdb

RegEx Demo

RegEx Details:

• (?<!a): Assert that previous character is not a
• a: Match single a
• (?=(?:aa)*(?!a)): Assert that we have 0 or more double a characters ahead that are not followed by another a