Multiline mode of anchors ^ $, flag "m"

The multiline mode is enabled by the flag m.
It only affects the behavior of ^ and $.
In the multiline mode they match not only at the beginning and the end of the string, but also at start/end of line.

Searching at line start ^

In the example below the text has multiple lines. The pattern /^\d/gm takes a digit from the beginning of each line:

let str = `1st place: Winnie
2nd place: Piglet
3rd place: Eeyore`;

console.log( str.match(/^\d/gm) ); // 1, 2, 3

Without the flag m only the first digit is matched:

let str = `1st place: Winnie
2nd place: Piglet
3rd place: Eeyore`;

console.log( str.match(/^\d/g) ); // 1

That’s because by default a caret ^ only matches at the beginning of the text, and in the multiline mode – at the start of any line.

Please note:

“Start of a line” formally means “immediately after a line break”: the test ^ in multiline mode matches at all positions preceded by a newline character \n.
And at the text start.

Searching at line end $

The dollar sign $ behaves similarly.
The regular expression \d$ finds the last digit in every line

let str = `Winnie: 1
Piglet: 2
Eeyore: 3`;

console.log( str.match(/\d$/gm) ); // 1,2,3

Without the flag m, the dollar $ would only match the end of the whole text, so only the very last digit would be found.

Please note:

“End of a line” formally means “immediately before a line break”: the test $ in multiline mode matches at all positions succeeded by a newline character \n.
And at the text end.

Searching for \n instead of ^ $

To find a newline, we can use not only anchors ^ and $, but also the newline character \n.
What’s the difference? Let’s see an example.
Here we search for \d\n instead of \d$:

let str = `Winnie: 1
Piglet: 2
Eeyore: 3`;

console.log( str.match(/\d\n/g) ); // 1\n,2\n

As we can see, there are 2 matches instead of 3.
That’s because there’s no newline after 3 (there’s text end though, so it matches $).
Another difference: now every match includes a newline character \n. Unlike the anchors ^ $, that only test the condition (start/end of a line), \n is a character, so it becomes a part of the result.
So, a \n in the pattern is used when we need newline characters in the result, while anchors are used to find something at the beginning/end of a line.

Original Content at: https://javascript.info/regexp-multiline-mode

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