[rt-users] custom fields validation

[Marco Avvisano]

I would like to have a custom field alphanumeric min 1 max 8 char
I try to use:

 (?#alfanum max 8)^[a-zA-Z0-9]{1,8} $

but don't work. 
i know very little regular expression ..
what's is wrong in my rexp?

thanks for help,

Marco