use strict;
my $str = '1
12
123
12345678
123456789
1 00000000
2 0000000
3 000000
4 00000
5 0000
6 000
7 00
8 0';
my $regex = qr/^((?|[1-9](?=.*1\t+(?<x>0+))|[1-9]\d(?=.*2\t+(?<x>0+))|[1-9]\d{2}(?=.*3\t+(?<x>0+))|[1-9]\d{3}(?=.*4\t+(?<x>0+))|[1-9]\d{4}(?=.*5\t+(?<x>0+))|[1-9]\d{5}(?=.*6\t+(?<x>0+))|[1-9]\d{6}(?=.*7\t+(?<x>0+))|[1-9]\d{7}(?=.*8\t+(?<x>0+))))\b/smp;
my $subst = '${x}$1';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html