5) write a regular expression would zip code + 4 fields of the forms: - where x is any number but the last 4 digits could be missing. the dash at the end of the first 5 numbers will always be present and the field is delineated with spaces or tabs.

Following is the expression written in "Bash script" (mixture of commands):

^\d{5}(?:[-\s]\d{4})?$

Explanation:

^ = For staring string (denotation). \d{5} = Matching 5 digits (first five unknown x's) (?:…) = Making Group (making group of former 5 digits)[-\s] = Match a hyphen  or a space

                 (checking if a hyphen is present?)

\d{4} = Matching 4 digits (next four unknown x's) …? = Pattern before it is optional $ = Ending of the string.(denotation)