What does RegExp.$1 do

Question

I have come across a piece of code in JScript:

RegExp.$1

Does anybody know what it does?

If I output it on its own, I get nothing not e

Answer 1

$1 is whatever is matched in the first capture. If you have more captures you can use $2, $3 etc.

Ex:

"abc".replace(/(.)/, "$1$1"); // aabc
"abc".replace(/(.{2})/, "$1$1"); // ababc
"abc".replace(/(.)(.)/, "$2$1"); // bac