Lots of things you can do with backreferences. Even check for primes: 
perl -wle 'print "Prime" if (1 x shift) !~ /^1?$|^(11+?)\1+$/'

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(by Abigail)

In reply to Re^2: Using regex to match double letters, and only double letters by ysth
in thread Using regex to match double letters, and only double letters by Anonymous Monk

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