[Solved] I want regular expression (theory of automata)

Question

Going completely classic regular expression (i.e. disjunction, concatenation, and kleene star):

(799|(8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)*|(1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)*)

If you allow typical regex shorthands (but stay within the theoretical limits, i.e. a regular language), that can be reduced to:

799|[89]\d{2,}|[1-9]\d{3,}

You match either the number 799, a three digit number starting with 8 or 9, or a four- (or more) -digit number starting with any digit but a 0 (to disallow 0023 matching).

Accepted Answer

Going completely classic regular expression (i.e. disjunction, concatenation, and kleene star):

(799|(8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)*|(1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)*)

If you allow typical regex shorthands (but stay within the theoretical limits, i.e. a regular language), that can be reduced to:

799|[89]\d{2,}|[1-9]\d{3,}

You match either the number 799, a three digit number starting with 8 or 9, or a four- (or more) -digit number starting with any digit but a 0 (to disallow 0023 matching).