Regex replace using pattern

I need to replace a pattern matched by regex with another pattern using regex, in C++.

Example –
We have the following characters: "a" and "b"

I want to replace like this –

Original text –

aabaaaaaaabaaabab

Replacement –

abbabbbbbbbabbbab

Replace logic –

"aab" must be replaced by "abb",

"aaab" must be replaced by "abbb",

"aaaab" must be replaced by "abbbb",

and so on…

I found the following regex for getting the matches –

aa+b

What regex replace pattern must be applied to get the desired replacement?

Thanks.

>Solution :

If using lookarounds be a possibility for you, here is one solution. Match on the following pattern:

(?<=a)a(?=a*b)

and then replace with just a single b. The pattern says to match:

(?<=a)   assert that at least one 'a' precedes (i.e. ignore first 'a')
a        a letter 'a'
(?=a*b)  which is following by zero or more 'a' then a 'b'

Here is working demo.