2.6 KiB
Multiline mode of anchors ^ $, flag "m"
The multiline mode is enabled by the flag pattern:m.
It only affects the behavior of pattern:^ and pattern:$.
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 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 pattern: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 pattern:^ 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 `pattern:^` 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 pattern:$ behaves similarly.
The regular expression pattern:\d$ finds the last digit in every line
let str = `Winnie: 1
Piglet: 2
Eeyore: 3`;
console.log( str.match(/\d$/gm) ); // 1,2,3
Without the flag pattern:m, the dollar pattern:$ 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 `pattern:$` 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 pattern:^ and pattern:$, but also the newline character \n.
What's the difference? Let's see an example.
Here we search for pattern:\d\n instead of pattern:\d$:
let str = `Winnie: 1
Piglet: 2
Eeyore: 3`;
console.log( str.match(/\d\n/gm) ); // 1\n,2\n
As we can see, there are 2 matches instead of 3.
That's because there's no newline after subject:3 (there's text end though, so it matches pattern:$).
Another difference: now every match includes a newline character match:\n. Unlike the anchors pattern:^ pattern:$, 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.