问Javascript regex“无效组”？

EN

' ^(?=.{4,16}$)(?![_.])(?!.*[_.]{2})[a-zA-Z0-9._]+(?<![_.])$
'  └─────┬────┘└───┬──┘└─────┬─────┘└─────┬─────┘ └───┬───┘
'        │         │         │            │           no _ or . at the end
'        │         │         │            │
'        │         │         │            allowed characters
'        │         │         │
'        │         │         no __ or _. or ._ or .. inside
'        │         │
'        │         no _ or . at the beginning
'        │
'        username is 4-16 characters long

[ERROR] :  Error generating AST for "***register.js"
[ERROR] :  Invalid regular expression: /^(?=.{4,16}$)(?![_.])(?!.*[_.]{2})[a-zA-Z0-9._]+(?<![_.])$/: Invalid group
[ERROR] :  Alloy compiler failed

var regex = /^(?=.{4,16}$)(?![_.])(?!.*[_.]{2})[a-zA-Z0-9._]+(?<![_.])$/;

        if ( !regex.test(e.value)) 
         {
            inputs.Username.borderColor = 'red';
            inputs.Username.backgroundColor = '#edcaca';
            return false;
         }