function isRFC822ValidEmail(sEmail) {

  var sQtext = '[^\\x0d\\x22\\x5c\\x80-\\xff]';
  var sDtext = '[^\\x0d\\x5b-\\x5d\\x80-\\xff]';
  var sAtom = '[^\\x00-\\x20\\x22\\x28\\x29\\x2c\\x2e\\x3a-\\x3c\\x3e\\x40\\x5b-\\x5d\\x7f-\\xff]+';
  var sQuotedPair = '\\x5c[\\x00-\\x7f]';
  var sDomainLiteral = '\\x5b(' + sDtext + '|' + sQuotedPair + ')*\\x5d';
  var sQuotedString = '\\x22(' + sQtext + '|' + sQuotedPair + ')*\\x22';
  var sDomain_ref = sAtom;
  var sSubDomain = '(' + sDomain_ref + '|' + sDomainLiteral + ')';
  var sWord = '(' + sAtom + '|' + sQuotedString + ')';
  var sDomain = sSubDomain + '(\\x2e' + sSubDomain + ')*';
  var sLocalPart = sWord + '(\\x2e' + sWord + ')*';
  var sAddrSpec = sLocalPart + '\\x40' + sDomain; // complete RFC822 email address spec
  var sValidEmail = '^' + sAddrSpec + '$'; // as whole string
  
  var reValidEmail = new RegExp(sValidEmail);
  
  if (reValidEmail.test(sEmail)) {
    return true;
  }
  
  return false;
}


/* example usage...
Using the function below, with html like this:
 <form method=post name="myform" action="/myscript.php">
 Email:<input type=text name="email" value="" size="30" />
 <input type="submit" name="submit" value="Submit" onClick="return checkEmailField(document.myform.elements['email']);" />
 </form>
*/

function checkEmailField(emailField) {
  var email = emailField.value;
  if (!isRFC822ValidEmail(email)) {
    alert("Please enter a valid email address.");
    emailField.focus();
    return false;
  }

  return true;
}