MySQL's regexp functions are limited to matching patterns, PostgreSQL can also use them to fetch substrings, and to replace substrings.
For example; to remove any spaces from a dutch zipcode:
SELECT regexp_replace(' 134 5 AA ', E'\\s', '', 'g')
That is: replace any whitespace character with an empty string, globally.
This way you can write a CHECK constraint, trigger or rule that accepts a zipcode in the raw format, and validate and clean it before storing it, making life a little easier on the application.