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JavaScript

function isEmail(email) {
  var regex = /^([a-zA-Z0-9_\.\-\+])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
  alert("email is "+regex.test(email));
  return regex.test(email);
}

isEmail('[email protected]')