var rx = /(^@|,$)/;

console.log(rx.test());    // false

console.log(rx.test(aaa)); // false

console.log(rx.test(@aa)); // true

console.log(rx.test(aa,)); // true

console.log(rx.test(@a,)); // true

var strings = [

  ,

  aaa,

  @aa,

  aa,,

  @a,

];

for (var i = 0; i < strings.length; i++) {

  var string = strings[i],

    result = string.length > 0 && (string.charAt(0) == @ || string.slice(-1) == ,);

  console.log(string, result);

}