[TYPO3-commerce] evaluation of the 'zip' form field

[Heiko Hänsge]

Hello,

I would like to evaluate the 'zip' form field of customer addresses.
I saw that the email address field is getting evaluated.
So would the pattern fit for the 'zip' field too?

plugin.tx_commerce_pi3.billing.sourceFields.zip.mandatory = 1
plugin.tx_commerce_pi3.billing.sourceFields.zip.eval =

And if yes which evaluation pattern/expression has to be applied to 
check for numbers/integers entered?
Thanks in advance for some hints.

regards
Heiko