What is the best way to iterate over string chars?

[Cooler]

Example1:
for(size_t i; i < s.length; ++i){
   // do something with s[i]
}

Example2:
foreach(i; 0 .. s.length){
   // do something with s[i]
}

Example3:
foreach(i, c; s){
   // do something with c and i
}

What is the preferable way for portability, performance, compiler?