It is nice, but it broke the rules :^( The post says no characters other then (),?,|,* this rules out the character class [] I believe. Some body please correct me if I am wrong. My own version that breaks the rules by using \d and not using the regex itself for success, but still a fun brain exercise is this:

S:while(<DATA>){%d=();while(/(\d)/g){if($d{$1})
{print "DUP: $1 in $_";next S}$d{$1}=1}} 

__DATA__
12345
012345
894376
3885437
98374
30924
03284098325
094385098432
74362
876543
4456789
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