$($b$)(2$ pts$)$ A genome sequence consists of the letters A$,$ C$,$ T$,$ G$.$ A potential gene is represented by a string of the form $($prefix$)$ gene $($postfix$),$ where prefix is the substring "ATG" and postfix could be one of the substrings "TAG", "TAA", and "TGA". Write a regex called 'pattern' representing a potential gene.