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Computer Science

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    $\begingroup$ If all else fails, you can always construct three separate regular expressions $R_1$, $R_2$, $R_3$ that correspond respectively to the automata where just the first, second and third state is accepting. And then take $R_1+R_2+R_3$. $\endgroup$ Commented May 5, 2019 at 14:14
  • $\begingroup$ Note that you can have arbitrarily long blocks of bs alternating with blocks of one or two as. $\endgroup$ Commented May 5, 2019 at 14:25