Exercise $7\mathrm{.}3-$ validate social security number

Define a function to validate the basic social security number format without using regular expression.

Social security basic format is ddd$-$dd$-$dddd $(3$ dights, hyphen, $2$ dights, hyphen, $4$ digits$)$

Write test code to ask for a social security number

and print out whether it is in the right format.

Challenge: Rewrite the function using regular expression.

Sample output:

Enter a string for SSN: $123-11-4567$

Valid SSN

Enter a string for SSN: $1234-1-123$

Invalid SSN