It's actually possible to avoid matching the extra character by using (?>...).

for my $d qw(2005-12 2005-100 2005-1- 2005-12- 2005-123- 2005-1000) {
    printf "%-10s: ", $d;
    if ( $d =~ /(?>\d{4}-\d{1,3})(?!-)/ ) {
      print "yep\n" ;
    } else {
      print "nope\n";
    }
}
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2005-12   : yep
2005-100  : yep
2005-1-   : nope
2005-12-  : nope
2005-123- : nope
2005-1000 : yep
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Use (?![-\d]) if you don't want the last to match.