Can you? I can, too:

perl -pe 's/cat/++$i/ge' file
[download]

There's no special variable that counts matches. But there's one that's incremented with each line of input: $.. So if we could tell Perl that cat instead of a newline is the line delimiter... but of course we can do that! That's what $/ is for.

perl -pe 'BEGIN { $/ = "cat" } chomp; $_ .= $. unless eof' file
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Update: which can be rewritten to use substitution as you originally wanted:

perl -pe 'BEGIN { $/ = "cat" } s{$/}{$.}' file
[download]

> > OF COURSE I can do this in a loop.

> s/cat/++$i/ge

interestingly it's possible to avoid /e in an efficient inside-out (TIMTOW) loop version of m///

say pos($txt) while $txt =~ m/cat/g

Now with s/// this kind of works, but isn't efficient because pos will be reset each time

use v5.12;
use warnings;
use Data::Dump qw/pp dd/;

my $txt = <<'___';
cat
dog
cat
mouse
eel
cat
housecat
catamaran
fish
___


say 'm-pos:', pos($txt) while $txt =~ m/cat/g;


my $cnt=1;

while ($txt =~ s(cat)($cnt)x) {
    say "s-pos:", pos($txt) // "undefined";
    $cnt++;
}

say $txt;
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m-pos:3
m-pos:11
m-pos:25
m-pos:34
m-pos:38
s-pos:undefined
s-pos:undefined
s-pos:undefined
s-pos:undefined
s-pos:undefined
1
dog
2
mouse
eel
3
house4
5amaran
fish
[download]

and no, using s///g would do the outer loop only once, tho there might be a way to force a single replace with (?FAIL) ... probably?

Cheers Rolf
_{(addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)
Wikisyntax for the Monastery}

I want a substitution with a Perlvar of the nth match, if one exists?

If I understand your question correctly, then no, no such perlvar that keeps track of "how many" regexes have been executed/replaced exists, and you'll have to use one of the other methods you described. Update: Or the clever hack by choroba.

Use /e to execute code to generate the substitution.

s{ cat }{ ++$i }xeg
[download]

Perhaps a programmatic pattern?

johngg@aleatico:~/perl/Monks$ perl -Mstrict -Mwarnings -E 'say q{};
my $text = <<__EOT__;
cat
dog
cat
mouse
eel
cat
housecat
catamaran
fish
__EOT__

my $n = 0;
$text =~ s{cat(?{ $n++ })}{$n}g;
say $text;'

1
dog
2
mouse
eel
3
house4
5amaran
fish
[download]

I hope this is helpful.

Just to add a note to this: while it may work in this case, in the general case, one should be careful with assuming how often a (?{...}) gets called, as is stated at the top of its documentation.

It isn't missing the blade. The blade is /e and you've just chosen not to use it for some undeclared reason.

---

🦛