From perlfaq4: How do I change the Nth occurrence of something?

For example:

$ perl -le'
my $x = "ababababab";
print $x;
my $count = 0;
$x =~ s/a/ ++$count <= 3 ? "B" : "a" /eg;
print $x;
'
ababababab
BbBbBbabab
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$_ = 'a a a a a a a a';
my $i = 3;
while($i){
   $i--;
   s/a/B/;
}
print "$_\n";
__END__
B B B a a a a a
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or, a little more concisely...

$s =~ s/a/B/ for(1..3);
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I think ig's reply and also Re: Does Perl support a given number of replacements ? have a bug. Consider:

>perl -wMstrict -le
"my $x = 'aBaCaDaEaFa';
 $x =~ s/a/aa/ for 1 .. 3;
 print $x;
"
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If I run this code, I might expect  'aaBaaCaaDaEaFa' to be printed, but I get  'aaaaBaCaDaEaFa' instead. This is because the regex of the substitution does not 'remember' its position in the string being searched, but instead starts from the beginning of the string on each invocation. So, if the substituted portion of the string contains anything that may be matched by the regex, then it, and not anything further along in the string, is what will be (repeatedly) replaced.

jwkrahn's reply Re: Does Perl support a given number of replacements ? shows a way around this.

Update: Improved first example and changed pertinent discussion, removed a second example as no longer offering greater clarity.

Wonderful !!! That's exactly what I was thinking of :)) Thanks.

A nice one. I use a similar code with awk. I had the impression that Perl might have a ready-made construct that works right out of the box. Thanks.

There is a catch though. If 'B' contains 'a', (or the replacement of an 'a' to a 'B' creates a new 'a'), this doesn't quite do what you want. A simple case, assume 'a' is '\w' and 'B' is just 'B'. Then

$_ = 'pqrstuvw';
my $i = 3;
while($i) {
  $i--;
  s/\w/B/;
}
print "$_\n";
__END__
Bqrstuvw
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$x = join 'B', split /a/, $x, 4;
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$x = 'ababababab';
$i=3;
$x =~ s/(^|[^a])a([^a]|$)/$1B$2/ while $i--;
print "$x\n"
__END__

BbBbBbabab
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Hi again, Thank you all for your time and solutions, I see that the problem becomes trickier when it comes to RE's ..., but at least now I know how Perl handles this situation. Appreciate your help. Thanks.