Own messages for constraints?

Hi !

Is it possible in postgres to define own message for some constraint?

Example:
If i create check constraint on zip_code column , i want to get
a message: "Zip code is invalid.Please,input code in format: nn-nnn"
and I want to send this message to the end user.
It will be much better(in my case) than "violates constraint
zip_code_check" :-) .

I can make this validation in trigger (and send msg to application by
notice or raise),but in this case i will duplicate validation
rules (in trigger and in constraint).Effect: Lower performance(i think)
and rules in two places...

Best regards and thanks for help,
Kacper Chrapa

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