Schema DEV

Hello, my name is Uriel Nusenbaum I´m from Buenos Aires, Argentina.
I need to make an a schema that use my own data type ? it's posible ?
for example :
  <element name = "FirstName" type = "ownstring" />
i like to define that ownstring data type is a set of characters but only letters and numbers : ex.: hggdff655
I will aprecciate the answer.
Thanks.