Regular expression and explain your answer.

a$.[$Points $5]$ Write a regex to match filenames that end with $.$txt or $.$pdf or $.$rtf$.$

b$.[$Points $5]$ Write a regex to match dates in the format DD$-$MM$-$YYYY or DD$/$MM$/$YYYY$.$

c$.[$Points $10]$ Write a regex to extract US phone numbers from a text. The phone numbers can be in the format $(123)456-7890$ or $123-456-7890$