regex - Javascript Email validation that allows a null value -

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i have on submit function checks email field have formatted address. works fine. however, want field allow null value particular input optional , not required. know should simple, im missing obvious. point me in right direction...

edit: providing entire form validation time. every time replace email portion breaking other pieces....

function validatesmsform() { if (smsform.pid_form.value.length < 4) { alert("please enter valid partner id"); return false; } if (smsform.area.value.length < 3) { alert("please enter valid 10-digit cell phone number"); return false; } if (smsform.prefix.value.length < 3) { alert("please enter valid 10-digit cell phone number"); return false; } if (smsform.line.value.length < 4) { alert("please enter valid 10-digit cell phone number"); return false; }  <!-- email validation here   if (/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/.test(form.emailaddress.value))       {       return (true)       }       alert("please enter valid email address")       return (false)   --> }

like this

assuming

<form onsubmit="return validatesmsform(this)">

javascript:

function isemail(str) {   return !str || /^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,})+$/.test(str); }  function validatesmsform(smsform) {   if (smsform.pid_form.value.length < 4) {     alert("please enter valid partner id");     return false;   }    if (smsform.area.value.length < 3 ||       smsform.prefix.value.length < 3 ||        smsform.line.value.length < 4) {     alert("please enter valid 10-digit cell phone number");     return false;   }     if (!isemail(smsform.emailaddress.value)) {     alert("please enter valid email address")       return false; //    }    return true; // allow submission }

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