Expression - Zip Code validation

We have the field called Zip Code and this is a text field and 10 Chrs long.

Field 1: State

Field u2018Stateu2019 is a lookup field and the lookup table has Country, state code and state name.

Field 2: Zip Code

Zip Code = text field and the length is 10 chrs.

The ZIP Code Validation should be either NNNNN format (Block of 5 digits) for standard ZIP Code OR

NNNNN-NNNN format (Block of 5 and 4 digits) for ZIP+4 format

For example, if the country = US and state = IL then the first digit of the postal code field will be 6 then remaining 4 digits must be numeric for the standard 5 digit Zip code.

Can anybody help me to write the expression. Full points will be rewarded.