console.dir( /(\w\d)(\w\d)(\w\d)/.exec('a1b2c3d4e5f6g7h8i9') );
// 0: "a1b2c3"
// 1: "a1"
// 2: "b2"
// 3: "c3"
// groups: undefined
// index: 0
// input: "a1b2c3d4e5f6g7h8i9"
// length: 4

console.dir( /(a1) (b2) \1 \2/.exec('a1 b2 a1 b2') );
// 0: "a1 b2 a1 b2"
// 1: "a1"
// 2: "b2"
// groups: undefined
// index: 0
// input: "a1 b2 a1 b2"
// length: 3

console.log( 'a1b2c3'.replace(/(\w\d)(\w\d)(\w\d)/, '$3,$2,$1') );
// c3,b2,a1

console.log( 'a1b2c3'.replace(/(\w\d)(\w\d)(\w\d)/, '($&)') );
// (a1b2c3)