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Which of the following is not a Regular Expression?

  1. $[(a+b)^* – (aa+bb)]^*$
  2. $[(0+1) – (0b + a1)^* (a+b)]^*$
  3. $(01+11+10)^*$
  4. $(1+2+0)^* (1+2)^*$
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Option A means set of all strongs over a and b except 2 consecutive a's or b's.

Option C is a  Regular expression if language consists 0,1 as alphabets.

Option D is a Regular expression if language consists 0,1,2 as alphabets.

But option B have no meaning as (0+1) - (0b + a1)* have no meaning hence this option is not a Regular Expression.
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