var input = "aaa@bbb.com (A,B); ccc@ddd.com (C,D); eee@fff.com (E,F);";
var initials = "(A,B)";
var email = input.match(new RegExp("([a-zA-Z0-9]+(?:[-._][a-zA-Z0-9]+)*@[a-zA-Z0-9]+(?:[-.][a-zA-Z0-9]+)*)(?=\\s*" + initials.replace(/([()])/g, "\\$1") + ")"))[0];
print(email);
dmFyIGlucHV0ID0gImFhYUBiYmIuY29tIChBLEIpOyBjY2NAZGRkLmNvbSAoQyxEKTsgZWVlQGZmZi5jb20gKEUsRik7IjsKdmFyIGluaXRpYWxzID0gIihBLEIpIjsKdmFyIGVtYWlsID0gaW5wdXQubWF0Y2gobmV3IFJlZ0V4cCgiKFthLXpBLVowLTldKyg/OlstLl9dW2EtekEtWjAtOV0rKSpAW2EtekEtWjAtOV0rKD86Wy0uXVthLXpBLVowLTldKykqKSg/PVxccyoiICsgaW5pdGlhbHMucmVwbGFjZSgvKFsoKV0pL2csICJcXCQxIikgKyAiKSIpKVswXTsKcHJpbnQoZW1haWwpOw==