8th May 2020

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`;

alert( 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`;

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.

“Start of a line” formally means “immediately after a line break”: the test ^ in multiline mode matches at all positions preceeded 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`;

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

“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`;

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.

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