JavaScript RegExp m Flag

What Is the m Flag?

The m flag (multiline) changes how ^ and $ work. They match the start and end of each line, not only the start and end of the whole string—ideal for logs, configs, and multi-line input.

/pattern/m           // multiline anchors
regex.multiline      // true when m is set

without m:  ^ = start of string,  $ = end of string
with m:     ^ = start of each line, $ = end of each line

m does NOT make . match newlines  (that is the s / dotAll flag)
combine: /^.+$/gm

How to Use the m Flag

Add m whenever your pattern uses ^ or $ per line.

1. Create a multiline regex: /^…/m, /…$/m, or new RegExp("^…", "m").
2. Put newlines in the string (\n) so line boundaries exist.
3. Add g when you need every line: /^…/gm.
4. Confirm with regex.multiline when debugging.

Pseudocode

re = /^pattern$/m
text has lines split by "\n"

if m is off:
    ^ only at index 0
    $ only at last index

if m is on:
    ^ also after each "\n"
    $ also before each "\n" (and at end)

test / match as usual

Cheat Sheet

Keep these multiline facts handy.

Live Preview

Compare anchors with and without m on multi-line sample text.

text = "nope\\nstart here\\nstart again"
/^start/m.test(text)
→ true

/* m lets ^ match after each newline */

JavaScript Examples

Short, copy-ready snippets. Open DevTools Console, or use Try It Yourself for a live editor.

const text = "nope\nstart here\nstart again";

console.log(/^start/.test(text));     // false
console.log(/^start/m.test(text));    // true
console.log(text.match(/^start/gm));  // ["start", "start"]
console.log(/^start/m.multiline);     // true

false
true
[ 'start', 'start' ]
true

const text = "line one end\nline two end\nno match";

console.log(/end$/.test(text));     // false
console.log(/end$/m.test(text));    // true
console.log(text.match(/end$/gm));  // ["end", "end"]

false
true
[ 'end', 'end' ]

const text = "apple\nbanana\ncherry";
const lines = text.match(/^.+$/gm);

console.log(lines);         // ["apple", "banana", "cherry"]
console.log(lines.length);  // 3

// Comment lines in a config:
const cfg = "# title\nname=js\n# end";
console.log(cfg.match(/^#.*/gm));  // ["# title", "# end"]

[ 'apple', 'banana', 'cherry' ]
3
[ '# title', '# end' ]

Performance Notes

Multiline matching still walks the string once. Prefer /^…/gm for simple line filters; use split when you will process every line with more than a regex.

Key Takeaways