SOURCE

var str = '123456'
var reg = /(1(2)3)(4(5)6)/

let result = str.replace(reg, function ($0, $1, $2, $3, $4, $5, $6) {
  console.log($0, $1, $2, $3, $4, $5, $6)
  return $1
})

console.log('结果：' + str.replace(reg, '$1'))