regex wrong in asp.net

i have :
 
<asp:TextBox ID="textBox2" runat="server" OnTextChanged="textBox2_TextChanged" Font-Size="Large" ValidationGroup="RegularExpressionValidator1"></asp:TextBox><br />
   
<asp:RegularExpressionValidator
      ID="RegularExpressionValidator1"
      runat="server"
      ControlToValidate="TextBox2"

      Display="Dynamic"
      ErrorMessage="(16 chars, only alphanumeric and underscore)"
      ValidationExpression="^[0-9a-zA-Z_]{1,16}$"
      ForeColor="Maroon"></asp:RegularExpressionValidator><br />



this for some reason


a) allows a single space as a valid result
b) doesn't throw an error when the user simply presses enter


any ideas? i have looked at the regex in regex bud ...