Claim: Every regular language is context free.

Proof: Let R be a regular expression that decides language L. We want to show that R can be converted to a context free grammar.
A regular expression has 3 operations to consider: union, concatenation, and star. The following shows how to convert each operation.

Union, between 2 languages with start states A and T respectively

S -> A | T

Concatenation, between 2 languages with start states A and T respectively

S -> AT | TA

Star, with a language with start state S

S -> SS | e