It´s a nice problem... :-)
I guess an easy way might be to split the text at desired char using substring (we know the index where we want to split) to then apply String method replaceFirst() on the second chunk and get them joined back again. Maybe there´re better ways using tough regEx rules, but I´m not good at regEx, don´t know... ;-)

Ale · 60rpm.tv/i

Mmm, it sounds really interesting... but Strings have replacing methods (replace(), replaceFirst()) so I don´t clearly understand what you say about them...

Ale · 60rpm.tv/i

You must be talking about this. Yeah, I've seen this, but unfortunately this doesn't do exactly what I need.

stringbuffer (or stringbuilder) seems to be a good choice. it converts the string to a char-array and then sets the new char at the specified index.
you can "extract" this functionality ( as you can see in "replaceCharAt_v1" ) to make it a bit faster, because you avoid using the stringbuffer class ... if its really just about replacing chars.

the following describe 3 different ways.

1. // using char-array (fastest)
2. // if you want to replace more chars in one string, you should create
3. // the char array outside the function
   
4. String replaceCharAt_v1(String s, int pos, char c) {
5.   char[] ch = new char[s.length()];
6.   s.getChars(0, ch.length, ch, 0);
7.   ch[pos] = c;
8.   return new String(ch);
9. }

1. // using stringbuilder (2nd fastest)
2. String replaceCharAt_v2(String s, int pos, char c) {
3.   StringBuilder sb = new StringBuilder(s); //or StringBuffer
   
4.   sb.setCharAt(pos, c);
5.   return sb.toString();
6. }

1. // using substring (slowest)
2. String replaceCharAt_v3(String s, int pos, char c) {
3.   return s.substring(0,pos) + c + s.substring(pos+1);
4. }

Sweet. Thanks for the great overview! Using the char-array seems like a nice way to do it.