You can't count in regexes, unless you actually call code blocks. This can be done either in the substitution part with s/.../.../e or in the regex with (?{ code }) assertions.

But all in all it seems foolish to do that kind of stuff in a single regex, because it's not what they are made for; hashes and a loop are a much better fit for this situation.

Update: nonetheless I tried to write a regex which does the substitution, but it only works for the first name unless you use it in a loop. Here it goes: 

use strict;
use warnings;

my $str = join '', <DATA>;

my $i = 0;
1 while $str =~ s{
    ^((?>\w+))(?!\d)(.*?)
    ^\1(?!\d)
}{"$1$2$1" . ++$i}msxeg;
print $str;

__DATA__
George Fortran
Jessi Heavens
Bill Clinton
Barack Obama
Bill Gates
Steve Jobs
Bill Green
George Bush

[download]

Second update: Uhm, I didn't read it too carefully, the code doesn't do what it should; ignore it...

In reply to Re: Indexing multiplicate firstnames with regex by moritz
in thread Indexing multiplicate firstnames with regex by stringZ

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