To correct give number to US E.164 format
Method: The method in this program takes user input (String) and compares first with regular expression and then replaces the string with desired format(E.164 format).
If the entered number is valid it formats and returns the formatted number, else it returns null.
Solution Decided:- example: if user enters : 1231231230 it formats to : (123) 123-1230
It is not mentioned for international or local. Hence, it is limited to number with area code as prefix.