Regex replace using pattern

user_18219276 0
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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.

regex
2 Answers

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.

user_18632290

Here is a solution that is not limited to the letters 'a' and 'b'.

Matches of the following regular expression are to be replaced by the content of capture group 2.

(?<=(.))\1(?=\1*(.))

Demo

The regular expression is comprised of the following elements.

(?<=    # begin a positive lookbehind
  (.)   # match any character and save to capture group 1
)       # end positive lookbehind  
\1      # match the content of capture group 1 
(?=     # begin a positive lookahead
  \1*   # match the content of capture group 1 zero or more times
  (.)   # match any character and save to capture group 2
)       # end positive lookahead
user_2569700
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