簡単なタスク - 正規表現^(GY|BT|JE)
または^(?:GY|BT|JE)
<script runat="server">
sub submit(sender As Object, e As EventArgs)
if Page.IsValid then
lbl.Text="OK"
else
lbl.Text="NOT OK"
end if
end sub
</script>
<!DOCTYPE html>
<html>
<body>
<form runat="server">Enter text here:
<asp:TextBox id="txtbox1" runat="server" />
<br /><br />
<asp:Button text="Submit" OnClick="submit" runat="server" />
<br /><br />
<asp:Label id="lbl" runat="server" />
<br />
<asp:RegularExpressionValidator
ControlToValidate="txtbox1"
ValidationExpression="^(GY|BT|JE)"
EnableClientScript="false"
ErrorMessage="Error"
runat="server" />
</form>
</body>
</html>