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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.




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