Javascript function for email validation

 Email Validator In JSF Using JavaScript
In Javascript write the following function

function emailcheck(str) {
    var at="@"
    var dot="."
    var lat=str.indexOf(at)
    var lstr=str.length
    var ldot=str.indexOf(dot)
    if (str.indexOf(at)==-1){
        alert("Invalid E-mail ID")
        return false
    }
    if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr){
        alert("Invalid E-mail ID")
        return false
    }
    if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
        alert("Invalid E-mail ID")
        return false
    }

    if (str.indexOf(at,(lat+1))!=-1){
        alert("Invalid E-mail ID")
        return false
    }

    if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot){
        alert("Invalid E-mail ID")
        return false
    }

    if (str.indexOf(dot,(lat+2))==-1){
        alert("Invalid E-mail ID")
        return false
    }

    if (str.indexOf(" ")!=-1){
        alert("Invalid E-mail ID")
        return false
    }

    return true
}

function validateUser()
{
    var flag = true;   
    var mailid = document.frmuser.tf_USER_MAILID;//frmuser is the form id

    if(mailid != null && mailid.value == "" ){
        flag = false;
        alert ("Mailid cannot be Empty...");
    }

    if (flag ) {
            flag =  emailcheck(mailid.value);
        }

    return flag;
}

In JSP:

Include javascript file into jsp using following code  
Call the javascript function using following code
<h:commandButton id="bt_SAVE" value="SAVE" onclick="javascript:return validateUser();" action="#{createuser.saveUserDetails}" style="font-size:16;font-weight:bold"/>

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