Help: Invalid character check

[Utech22]

What is wrong with :

function check ( char )
{
var valid = ( [^a-zA-Z 0-9] );
if (char.code.value == ""  || char.code.value != valid ) 
{
    alert( "Please enter a code ." );
    char.code.focus();
    return false ;
}
  return true ;
}