Your second way doesn't work if there's a bad match found before a good match can be found. You could embed some logic into the regex, but I'd really probably just use that first method of yours. Here's the steroid-enhanced way: 
my ($START, $END) = (11, 30);
if ($string =~ /^\C{$START,$END}?($pattern)(?(?{ $+[0] < $END })|(?!))
+/) {
  # it's ok
}

[download]
It matches (in the test case) between 11 and 30 characters at the beginning of the string, and then tries matching the pattern. If, after it matches the pattern, $+[0] is still less than $END, then it succeeds; otherwise, it backtracks.
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In reply to Re: Applying a regex to part of a string by japhy
in thread Applying a regex to part of a string by holdyourhorses

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