# Is The Following Language Regular? [duplicate]

Let $L_{1}$ and $L_{2}$ be 2 languages over the same alphabet $\Sigma$.

$$A(L_1,L_2)=\{x\in \Sigma^*|\exists y,z\in L_2\text{ such that } yxz\in L_1\}$$

Assume that $L_{1}$ is regular and $L_{2}$ is context-free. The language $A(L_{1},L_{2})$:

1. is always a regular language
2. is always not a regular language
3. can sometimes be a regular language
4. cannot be context free

They say that the correct answer is 1.

• – Ran G. Feb 28 '13 at 2:50
• Wow, thanks ! I'm shocked to see the answer to that question. – Robert777 Feb 28 '13 at 6:45
• @Robert777 So this is a duplicate? – Raphael Feb 28 '13 at 10:10
• @Raphael Pretty much. – Yuval Filmus Mar 1 '13 at 3:37

Hint: Take a DFA for $L_1$. Check which states are reachable from the initial state via a word in $L_2$. Check from which states a final state can be reached via a word in $L_2$.