Does Regex's [m] flag must come with [g] flag?

Question

I read on MDN that :

m flag  / multiline : 

Treat beginning and end characters (^ and $) as working over multiple lines

So I made a test (http://regexr.com?374jj) :

I have this simple regex :

^[\s\S]{3}

If I dont check global and multiline :

If I check only global :

If I check both global + multiline :

So it seems that multiline works only with the global flag.

Does my observation/conclusions are right ? Does multi line should be always with global ?

Answer 1

m doesn't require or imply g, no. Consider:

"foo\nbar".match(/^bar/)   // `null`

vs.

"foo\nbar".match(/^bar/m)  // ["bar"]

With the m flag, ^bar matches because the ^ matches at the beginning of the line. Without it, there is no match, because ^ doesn't match at the beginning of the input string. The m flag has the analogous effect on the end-of-(line|input) anchor $ as well.

The g flag comes into play when you need to do the match more than once. Consider this difference, for instance:

"foo\nbar\nfoo\nbar".replace(/^bar/m, "BAZ")

...which gives us:

foo
BAZ
foo
bar

Note that the second match wasn't replaced. Compare with the result if we add the g flag:

"foo\nbar\nfoo\nbar".replace(/^bar/mg, "BAZ")

...which gives us:

foo
BAZ
foo
BAZ

Note that all matches were replaced.

Answer 2

Well, it depends what you are trying to achieve. Without the global flag, you will only get the first match. Without the multiline flag, only the first line will be considered.