How to replace all characters except a specific one?

Question

JavaScript

Let's say I have a string +7(\999) 999 99 99

How to replace all 9 in the string with _ , except for the one preceded by the \ character , and remove the character itself, it can be anywhere.

The output should be +7(9__) ___ __ __

PS: 7(\999) 999 99 99 => +7(9__) ___ __ __

Answer 1

str.replace(/(\\?)(\d?)/g, (m, s, n) => s || n !== '9' ? n : '_')