Another way: 

my $nth = 3;
my $rep = 1;
my $chr = 'e';
my $str = "Terence and Philip are sweet\n";

my $pos = 0;
$pos = index($str, $chr, $pos+1) for(1..$nth);
substr($str, $pos) =~ s/$chr/$rep/g;

print $str;

[download]

You could, of course, move the $pos finding part into a do block within the substr expression but it's a bit unwieldy.

Update: Well, my method's a little bit more wordy then quent's but it's ever-so-slightly faster, almost definately because it avoids doing the string-eval. I don't think it's a significant difference, though. I was just curious. :-) 

my $s = "Terence and Philip are sweet\n";
my $c = 'e';
my $r = 1;
my $n = 3;

sub mine {
  my ($str, $chr, $rep, $nth) = @_;
  my $pos = 0;
  $pos = index($str, $chr, $pos+1) for(1..$nth);
  substr($str, $pos) =~ s/$chr/$rep/g;
  return $str;
}

sub quents {
  my ($str, $chr, $rep, $nth) = @_;
  my $i = 1;
  $str =~ s/($chr)/$i++<$nth?$1:$rep/eg;
  return $str;
}

use Benchmark qw( timethese cmpthese );

cmpthese(500000, {
  "Mine"    => sub { mine   ($s, $c, $r, $n) },
  "Quent's" => sub { quents ($s, $c, $r, $n) },
});

__END__
Benchmark: timing 500000 iterations of Mine, Quent's...
      Mine: 14 wallclock secs (14.08 usr +  0.00 sys = 14.08 CPU) @ 35
+511.36/s (n=500000)
   Quent's: 19 wallclock secs (18.58 usr +  0.03 sys = 18.61 CPU) @ 26
+867.28/s (n=500000)
           Rate Quent's    Mine
Quent's 26867/s      --    -24%
Mine    35511/s     32%      --

[download]

bbfu
Seasons don't fear The Reaper.
Nor do the wind, the sun, and the rain.
We can be like they are.

In reply to (bbfu) (another way) Re: Replacing a given character starting with the xth occurence in a string by bbfu
in thread Replacing a given character starting with the xth occurence in a string by Anonymous Monk

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