How can we add a regex to the customcontrol inherited from TextBox.

public override void RenderBeginTag(HtmlTextWriter writer)
        {            
            // Set the maxlength and the min length using regex for multiline textboxes...
            RegularExpressionValidator rev = new RegularExpressionValidator { ControlToValidate = ClientID, ValidationExpression = @"^[\s\S]{" + MinLength + "," + (MaxLength == 0 ? 1000000 : MaxLength) + "}$", ErrorMessage = "Please enter a valid length." };
            this.Parent.Controls.Add(rev);
// The prev line is giving error:
// Collection was modified; enumeration operation may not execute.
// Even tried the above in Onprerender also, but no luck.

            base.RenderBeginTag(writer);
        }