codice:
<form name="Form1" action="pagina.asp">
<select size="1" onchange="funMatr()" name="txtCombo" tabindex="5"> 
</form>
<SCRIPT LANGUAGE="VBSCRIPT"> 

Function funMatr() 
Val1 = Mid(Form1.txtCombo.Value, 1,18) 
Val2 = Mid(Form1.txtCombo.Value, 19,5) 

document.Write Val1 
document.Write Val2 

End Function 
</script>

prova cosi'