Although the exact answer to the question has been given, I feel there is a deeper answer to this. I would advise searching for what you DO expect rather than picking on what might be insufficient examples of what you don't. In this case, that isn't clear in the OP. But if the data is supposed to terminate at this point, it is better to match on the terminator, e.g. 
/^\d{4}\-\d{2}$/ or die; # match on end of string after \d{2}, or ...
/^\d{4}\-\d{2}\s+/ or die; # match on whitespace delimiter
# etc.

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-M

Free your mind

In reply to Re: negative look-ahead is ignored by Moron
in thread negative look-ahead is ignored by jeanluca

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