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Comment by elwell | Hacker News Reader

I figured out Ranges!

  abac|accede|adead|babe|bead|bebed|bedad|bedded|bedead|bedeaf|caba|caffa|dace|dade|daff|dead|deed|deface|faded|faff|feed

Edit: /s

Prime:

    ^x{2,3}$|^x{5}$|^x{7}$|^x{11}$|^x{13}$|^x{17}$|^x{19}$|^x{23}$|^x{29}$|^x{31}$|x{33}

josephlord's solution is much nicer:

^(?!(..+)(\1)+$)

Look at the parent.

[a-f]{4,} works as well... but I do like your solution!

[a-f]{4} is shorter.

  ^[a-f]*$

Is the same length too.