Failed Substring in XPATH Receiver Determination

Hi,

I have this payload.

<ns:request>

     <in>

          <materialno>AU1234</materialno>

     </in>

<ns:/request>

I'm using xpath in receiver determination and I have to check the country code prefix in the material number. I need to use Substring function and not ContainsPattern (double ~) because I will create a longer expression later on.

I'm using this condition but it doesn't work:

/ns:request/in[substring(materialno,1,2)='AU'] EX

Any idea? Appreciate your inputs!