# Regular Expression: L= {w | every even position of w is '1'}

I am trying to solve a regular expression of binary string where every even position is a '1'

I've solved this for an odd position: (1(0+1))*(1+ε)

How would it look like for an even position then? Thanks in advance.

• You successfully solved the odd position case. What makes the even position case conceptually more difficult? – Hendrik Jan Apr 21 '19 at 12:05
• I was a bit confused though. – Shunjid Rahman Apr 22 '19 at 7:51

An odd position can either have 0 or 1 and every even position can only have 1. RE for this can be: $$((0 + 1)1)^* (\epsilon + (0+1))$$ Note that, it also contains the empty word.