Are we talking ASP .NET bere ? if yes, there are two ways of doing it
1. Using JavaScript to call a function (See solution 1)
2. Using Regular Expressions Validation
<asp:regularexpressionvalidator id="valPassword" runat="server" xmlns:asp="#unknown">
ControlToValidate="txtPassword"
ErrorMessage="Minimum password length is 5"
ValidationExpression=".{5}.*" /</asp:regularexpressionvalidator>