How to replace string between two string with the same length

Using replace() and passing the match to a function as parameter, and then Array(m.length).join("X") to generate the X's needed:

var str = "ABCDEFG[HIJKLMN]OPQRSTUVWXYZ"

str = str.replace(/\[[A-Z]*\]/g,(m)=>"["+Array(m.length-1).join("X")+"]")

console.log(str);

We could use also .* instead of [A-Z] in the regex to match any character.

About regular expressions there are thousands of resources, specifically in JavaScript, you could see Regular Expressions MDN but the best way to learn, in my opinion, is practicing, I find regex101 useful.

const str="ABCDEFG[HIJKLMN]OPQRSTUVWXYZ";
const run=str=>str.replace(/\[.*]/,(a,b,c)=>c=a.replace(/[^\[\]]/g,x=>x="X"));
console.log(run(str));

The first pattern /\[.*]/ is to select letters inside bracket [] and the second pattern /[^\[\]]/ is to replace the letters to "X"

We can observe that every individual letter you wish to match is followed by a series of zero or more non-'[' characters, until a ']' is found. This is quite simple to express in JavaScript-friendly regex:

/[A-Z](?=[^\[]*\])/g

regex101 example

(?= ) is a "positive lookahead assertion"; it peeks ahead of the current matching point, without consuming characters, to verify its contents are matched. In this case, "[^[]*]" matches exactly what I described above.

Now you can substitute each [A-Z] matched with a single 'X'.

You can use the following solution to replace a string between two square brackets:

const rxp = /\[.*?\]/g;
"ABCDEFG[HIJKLMN]OPQRSTUVWXYZ".replace(rxp, (x) => {
    return x.replace(rxp, "X".repeat(x.length)-2);
});