为什么此RegExp无法正常工作？

Question

I made a RegExp to format an incoming date string, but it doesn't work as expected with my usage. I was hoping someone could explain why not:

 var data = [ "m_2013_01_01", "m_2013_02_01", "m_2013_03_01", "m_2013_04_01" ]; // why aren't these equivalent? // expected console.log(data.map(datum => datum.replace(/^m_(\\d+)_(\\d+)_(\\d+)/g, '$1-$2-$3'))); // ??? console.log(data.map(datum => datum.replace(/^m_(?:(\\d+)_?){3}$/g, '$1-$2-$3'))); 

Answer 1

In the fist regex you are using 3 groups:

That's why you can reference group 1, 2 and 3.

However, in the second regex you are using 1 group repeated multiple time, so group 2 and 3 doesn't exist and can't be referenced: