Enable Disable ASP.Net RequiredField Validator from Server Side (Code Behind) in ASP.Net

Refer this one.

HTML

<div>
    <asp:TextBox ID="txtPassword" runat="server" />
    <asp:CheckBox ID="ChkComplexPass" runat="server" AutoPostBack="true" OnCheckedChanged="ChkComplexPassOnCheckedChanged" />
    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" ErrorMessage="Required" ControlToValidate="txtPassword"
        runat="server" ValidationGroup="usrvalid" />
    <asp:RegularExpressionValidator ID="Regex4" runat="server" ControlToValidate="txtPassword"
        ValidationExpression="^(?=.*[A-Z])(?=.*\d)(?=.*[$@$!%*?&])[A-Za-z\d$@$!%*?&]{8,}"
        ErrorMessage="Password must contain atleast 1 UpperCase Alphabet, 1 Number and 1 Special Character"
        ForeColor="Red" ValidationGroup="usrvalid" />
    <br />
    <asp:Button ID="Button1" Text="Save" OnClick="Save" runat="server" ValidationGroup="usrvalid" />
</div>

 C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        this.PasswordPolicy();
    }
}

protected void ChkComplexPassOnCheckedChanged(object sender, EventArgs e)
{
    if (this.ChkComplexPass.Checked)
    {
        this.Regex4.Enabled = true;
    }
    else
    {
        this.Regex4.Enabled = false;
    }
}

private void PasswordPolicy()
{
    if (this.ChkComplexPass.Checked)
    {
        this.Regex4.Enabled = true;
    }
    else
    {
        this.Regex4.Enabled = false;
    }
}

protected void Save(object sender, EventArgs e)
{
    string s = string.Empty;
}

 You can also do it from Client Side

Refer