Is there a more idiomatic/elegant way to achieve the following, while remaining as efficient as possible? 

Same question in the simpler case n==0?

using retro seems inefficient because of all the decodings

// returns the largest suffix of a that contains no more than n times c

string findBack(string a, char c, size_t n=0){

  auto b=cast(immutable(ubyte)[])a;

  auto val=cast(ubyte)c;

  size_t counter=0;

  for(ptrdiff_t i=cast(ptrdiff_t)b.length - 1; i>=0; i--){

    if(b[i]==c){

      if(counter>=n)

        return cast(string)a[i+1..$];

      counter++;

    }

  }

  return a;

}

//unittest{

void test3(){

  auto c='\n';

  string a="abc1\nabc2\nabc3";

  assert(a.findBack(c,0)=="abc3");

  assert(a.findBack(c,1)=="abc2\nabc3");

  assert(a.findBack(c,2)=="abc1\nabc2\nabc3");

  assert(a.findBack(c,3)=="abc1\nabc2\nabc3");

}