Javascript uses regular expressions to implement digital thousand separators

recently on the Internet to see the use of JS to achieve the number of thousands of separators for the interview (written) questions, so I wrote a "regular +replace" to achieve the method:    

1 var function (numstr) {2     var b =/([-+]?\d{3}) (? =\d)/g; 3 4     return function ($ A, $) {5         return $ + ', '; 6     }); 7 }

Support sign matching, decimal distinction, if there are errors, we hope that the big point:-D

Javascript uses regular expressions to implement numeric thousand separators