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    $\begingroup$ You can always construct an NFA and convert it into a regular expression. You can first construct an NFA for all words containing bab or abb, then make it into a DFA, then complement it, then convert it into a regular expression. $\endgroup$ Commented Sep 23, 2017 at 22:43
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    $\begingroup$ Think of what string could precede $b$. For example, what could precede $bb$? In this way, try to come up with "building blocks" for strings not containing bab and abb. $\endgroup$ Commented Sep 23, 2017 at 22:46
  • $\begingroup$ "But putting them together is a huge animal" -- How do you "put together" regular expressions? $\endgroup$ Commented Sep 24, 2017 at 13:13
  • $\begingroup$ @Raphael we "put together" by concatenation, intersection, and union. $\endgroup$ Commented Sep 24, 2017 at 15:51
  • $\begingroup$ @fade2black I am not aware of a canonical regular expression operation that provides intersection. $\endgroup$ Commented Sep 24, 2017 at 17:07