HI amar,
Use below Regular Expression.
(?=.*[0-9])\d{0,3}(?:\.\d{0,1})?$
HTML
<asp:TextBox ID="TextBox1" runat="server" />
<br />
<asp:RegularExpressionValidator ID="Regex1" runat="server" ValidationExpression="(?=.*[0-9])\d{0,3}(?:\.\d{0,1})?$"
ErrorMessage="Please enter valid integer or decimal number." ControlToValidate="TextBox1" />
Output
Valid
0.0
999.0
Invalid
999.00
9990.0