patternMinor
Regular expression for strings not starting with 10
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expressionstartingwithregularstringsfornot
Problem
How can I construct a regular expression for the language over $\{0,1\}$ which is the complement of the language represented by the regular expression $10(0+1)^*$?
Solution
If a word doesn't start with $10$, then either it starts with some other combination of two letters, or it is shorter than two letters. All in all, we get the following regular expression for your language:
$$
\epsilon + 0 + 1 + (00+01+11)(0+1)^*.
$$
$$
\epsilon + 0 + 1 + (00+01+11)(0+1)^*.
$$
Context
StackExchange Computer Science Q#118174, answer score: 2
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