Regex Flags Explained — g, i, m, s, u, y in JavaScript

JavaScript Regex Flags

Flags modify how the regex engine processes your pattern. JavaScript supports six flags, each controlling a different aspect of matching behavior.

Flag Reference

Flag	Name	Effect
g	Global	Find all matches, not just the first
i	Case-insensitive	Ignore case when matching
m	Multiline	^ and $ match line boundaries
s	Dotall	. matches newline characters
u	Unicode	Enable full Unicode support
y	Sticky	Match only at lastIndex position

Global Flag (g)

Without g, the regex stops after the first match. With g, it finds all matches:

"banana".match(/a/g)  // ["a", "a", "a"]
"banana".match(/a/)   // ["a"] (only first)

Case-Insensitive Flag (i)

Makes the entire pattern case-insensitive:

/hello/i.test("Hello")  // true
/hello/i.test("HELLO")  // true

Multiline Flag (m)

Changes ^ and $ to match line boundaries instead of string boundaries:

"line1\nline2".match(/^line/gm)  // ["line", "line"]
"line1\nline2".match(/^line/g)   // ["line"] (first only)

Dotall Flag (s)

Makes . match newline characters (\n, \r):

/a.b/s.test("a\nb")   // true
/a.b/.test("a\nb")    // false

Unicode Flag (u)

Enables full Unicode support, including surrogate pairs and Unicode property escapes:

/\u{1F600}/u.test("\u{1F600}")  // true (emoji)
/\p{Letter}/u.test("\u{00E9}")     // true (accented letter)

Sticky Flag (y)

Matches only at the position indicated by lastIndex:

const re = /\d+/y;
re.lastIndex = 4;
re.exec("abc 123")  // matches "123" starting at index 4

Combining Flags

Flags can be combined: /pattern/gims. The most common combinations are gi (global, case-insensitive) and gm (global, multiline).