sorting zip codes

[Rich Lague]

I’m trying to set up a sort of a stack of address cards by zip code. 
Some of the zips are only the first 5 numbers and some of them are in 
the form “xxxxx-xxxx”.

The script, “sort cards numeric by field "zip"“ sorts all the 9 number 
zips first, then sorts the 5 number zips-- not what I want.

I tried “sort cards of this stack numeric by chars 1 to 5 of field 
“zip”” I go a “bad factor” error.

Then I tried “sort cards of this stack numeric by first 5 chars of 
field “zip”” Another “bad factor”.

I also tried, “sort cards of this stack numeric by (chars 1 to 5 of 
field "zip")” -- Bad factor still.

Is there a way?

Thanks,

Rich