Problem Detail: I have to write a regular expression that accepts any odd binary number not preceded by a 0. the best I can come up with is $1(0cup1)^*1$, but that doesn’t match just 1. The best it matches is 11.
Asked By : agent154
Answered By : Hendrik Jan
First you might just add that single $1$ to your expression as an alternative to what you already have. One obtains $1 cup 1(0cup 1)^*1$. An alternative is $1(0^*1)^*$. That one is shorter, but not necessarily better. Actually that expression I obtained by starting with the FSA below, and realizing that the rightmost two states could be replaced by iterating $0^*1$. 
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Question Source : http://cs.stackexchange.com/questions/10983
