Question

Why does javascript replace string function do this?

"aaa\nbbb\nccc".replace(/.*/gm, ".")
// result = "..\n..\n.." but expected was: ".\n.\n."

"aaa\nbbb\nccc".replace(/^.*/gm, ".")
// result = ".\n.\n." -> OK!!!

"aaa\nbbb\nccc".replace(/.*$/gm, ".")
// result = "..\n..\n.." but expected was: ".\n.\n."

What I am doing wrong?

Answer 1

Let me address those in reverse order:

What I am doing wrong?

You want to use +, not *. * means zero or more matches, which makes no sense here. + means one or more matches. So:

"aaa\nbbb\nccc".replace(/.+/g, ".")
// ".\n.\n."

Also note that if you’re not using ^ or $ (your first example), you don’t need the m modifier (but that wasn’t the problem with what you were doing). And you don’t need ^ or $ because . doesn’t match newlines (something I didn’t know prior to answering this question).

Why does javascript replace string function do this?

I have no Earthly idea and hope someone else does.

Again, by using * you’re saying zero or more matches. So it matches all of the relevant characters, replacing them with the first dot; then it matches zero characters, replacing them with one dot. Result: Two dots.

Proof:

Live copy | Live source

"aaa\nbbb\nccc".replace(/.*/g, function(m) {
    console.log("m = '" + m + "'");
});

Outputs:

m = 'aaa'
m = ''
m = 'bbb'
m = ''
m = 'ccc'
m = ''