If you want to write this in C I suggest getting a good beginners book. If you want to learn perl (since this is a perl forum) check out https://learn.perl.org, the Tutorials->Getting Started with Perl. If you have a perl problem you want help with please read and understand How do I post a question effectively?.

"... check out ..." perlintro also.

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Give a man a fish:  <%-{-{-{-<

- Do you want to write a C program or a Perl program? If you want to have C code, then you probably chose the wrong forum. Either way, you might want to start with good book for beginner and first try to solve your problem by yourself.

- I have no idea what "replace a charecter with another position based on its postion" means and your purported example is as limpid as mud. A good example should state the input, the output and often an explanation on how to get from the input to the output.

Please clarify.

Update: fixed a small typo.

Here is a tutorial about string manipulation. https://perlmaven.com/string-functions-length-lc-uc-index-substr