reverse a string in place

[Priti24]

I have a sring like $string = "abcdefghi"; i need to reverse the string in place i.e without using any other array or built-in function. The algorithm used for this is swapping the numbers like below:

function reverse_in_place(a[0..n])
     for i from 0 to floor(n/2)
         tmp := a[i]
         a[i] := a[n - 1 - i]
         a[n - 1 -; i] := tmp

Can you please tell me is there any regular expression that can be used to reverse the string or any perl code will be helpful. Thanks in advance.