Need to understand why the regex is not replacing all matches

Question

I am trying to figure out the following regex expression and why it is giving me the result I am getting.

I have the following javascript:

let result = '7979797'.replace(/797/g,'77');

I would have expected result to have the value of 7777 but instead it has a value of 77977.

I was hoping someone could explain why I am getting a value of 77977 and what I would need to change to the regex to get it replace all strings that have the patter 797 to 77.

Answer 1

When a regex replaces the first 797 with 77, it doesn't rescan the material it has replaced (the 77), so it sees 9 next, then 797, leading to the result you get.