Regular expression : I have this context free grammar :
S -> aSb

S -> aSa

S -> bSa

S -> bSb

S -> epsilon 
I want to show that this grammar describes a regular language ( namely can be represented as a regular expression) but I'm not sure how to do that and get the confident I'm not missing any pattern.
I did not see this exact question and that why I don't think it is duplicate. I'd like an explanation on this relative simple example. It was hard for me to follow more complicated examples.

