in reply to Numbering instances of a String in a file

use strict;
use warnings;

my $cnt = 1;
while (<>) {
    s/$/sprintf '%04d', $cnt++/e if /^\*+$/;
    print;
}

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Update: The above code functionally satisfies the OP specification. Thanks to ikegami for demonstrating several optimization techniques below.

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Re^2: Numbering instances of a String in a file
by ikegami (Patriarch) on Jun 07, 2010 at 17:33 UTC

    Needless sprintf, needless match, and you should probably separate the number from the stars with a space. I'd also require at least 3 stars to trigger a match. 

    my $cnt = '0000';
while (<>) {
    s/^(\*{3,})$/"$1 ". ++$cnt/e;
    print;
}

    [download]

    Optimised: (requires 5.10+) 

    my $cnt = '0000';
while (<>) {
    s/^\*{3,}\K$/" ". ++$cnt/e;
    print;
}

    [download]

    As a one-liner: (Second version requires 5.10+) 

    perl -pe'BEGIN { $cnt = '0000' }  s/^(\*{3,})$/"$1 ". ++$cnt/e'
perl -pe'BEGIN { $cnt = '0000' }  s/^\*{3,}\K$/" ". ++$cnt/e'

    [download]
[reply]
[d/l]
[select]
      Without an eval: 
      perl -ple 'BEGIN {$cnt = '0000'} $_ .= " " . ++$cnt if /\*{3,}$/'

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[reply]
[d/l]
        None of the solutions used an eval. That would be /ee.
[reply]

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