isValidEmail

isValidEmail.js

/**
 * Validates e-mail addresses
 * Supports internacionalized e-mails
 */
function isValidEmail(input) {
  // Split the e-mail into local part and domain part:
  var parts = input.split('@');
  
  // It needs to have those two parts:
  if (parts.length !== 2) return false;
  
  // Remove comments:
  var commentRegex = /(^\([^\)]*\)|\([^\)]*\)$)/g;
  parts[0] = parts[0].replace(commentRegex, '');
  parts[1] = parts[1].replace(commentRegex, '');
  
  // Ensure domain part and that it doesn't exceed 256 characters
  //  and consist only on valid characters:
  if (!parts[1] || parts[1].length >= 256 ||
  !(/^(?:[\u007F-\uffff]|[\w\-\.])+$/i.test(hostnameParts[i]))) return false;
  
  // Split the hostname into parts:
  var hostnameParts = parts[1].split('.');
  if (!hostnameParts.length) return;
  
  for (var i = 0; i < hostnameParts.length; i++) {
    // Each part needs to be between 1 to 63 characters
    if (!hostnameParts[i] || hostnameParts[i].length > 63) {
      return false;
    }
  }
  
  // The local part need to exist, can't start or end with a dot, neither have two consecurive dots:
  if (!parts[0] ||
      parts[0].charAt(0) === '.' ||
      parts[0].charAt(parts[0].length - 1) === '.' ||
      parts[0].indexOf('..') !== -1) {
    return false;
  }
  
  // "(),:;<>@[\] and spaces are only allowed between quotes:
  if (/[\s"(),:;<>@\[\\\]]/.test(parts[0].replace(/(^|\.)"((?:[^"\\]|\\"|\\)+)"((?=\.)|$)/g, '.'))) {
    return false;
  }
  
  // Otherwise it's a valid e-mail:
  return true;
}

New update has better rules for hostnames while still accepting IDNs. Maybe it's possible to improve the comment removing steps.

It doesn't validates e-mails with @ in the local part, like "@ 1145 jan 31"@example.com . As this function tries to follow the RFCs it needs to be fixed.

A warning: Gmail, which considers those e-mails valid, have bugs when sending e-mails to e-mails like that. Same for e-mails with < and > in the local part: seems the parser used to extract the address from the header doesn't handle those cases.