For a language $\text{L}$ over an alphabet $\Sigma$, define
$$\text{SW(L)}:= \left \{ y \in \Sigma^{\ast} \mid \exists x \in \Sigma^{\ast}\; \text{s.t.}\; xyx \in \text{L} \right \}$$
Prove that if $\text{L}$ is regular, $\text{SW(L)}$ is also regular.