The question asked for a regex solution. If you want to deviate and use a loop, there's no need to use a couple of regexes inside the loop. Something like (untested): 
perl -lne 'die "Line ", $.-2, "\n" if ($e = length() ? 0 : $e+1) == 2'

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In reply to Re^3: search for blank lines by RegEx [!] by Anonymous Monk
in thread search for blank lines by RegEx by mosh

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