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How to validate length of input into a multi-line textbox

By James Hippolite, in , posted: 26-Aug-2008 08:18

You can validate the length of input of an ordinary TextBox using the MaxLength attribute.  However, this doesn't work if the TextBox has a TextMode of Multi-line.  In those instances, use this piece of code.

<script type="text/javascript" language="javascript">


    function MaxLength()


        var txt = $get("<%=ResponseNote_TextBox.clientid %>");

        var msg = $get("<%=Message_Label.clientid %>");


        if (txt.value.length > 500)




            msg.className = "Failure";

            msg.innerHTML = "* Response has more than 500 characters";




            if (msg.innerHTML == "* Response has more than 500 characters")


                msg.innerHTML = "";





    function SelectText()


            var txt = $get("<%=ResponseNote_TextBox.clientid %>");

            var rng = txt.createTextRange();


            rng.moveStart("character", 501);

            rng.moveEnd("character", 0);




    function RemoveCRLFs()


   // debugger;

    var txt = $get("<%=ResponseNote_TextBox.clientid %>");

    var str = window.clipboardData.getData("Text");


    str = str.replace(/(\s*(\r?\n|\r))+$/, '');


    txt.value = str ;








ResponseNote_TextBox.Attributes.Add("onblur", "javascript:MaxLength()")

ResponseNote_TextBox.Attributes.Add("onpaste", "javascript:RemoveCRLFs()")


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JamesHip's profile

James Hippolite
New Zealand

Welcome to my technical blog. 

Here, I attempt to distill the Microsoft Certified Professional Developer knowledge I have accumulated since first qualifying MCP in 1996.  This blog started on 13 September 2007 as an off-shoot from my mixed up personal blog.  But it took a shot in the arm from Scott Hanselman's talk at TechEd New Zealand 08 "32 Ways To Make Your Blog Suck Less".