The unicode flag /.../u enables the correct support of surrogate pairs.

Surrogate pairs are explained in the chapter Strings.

Let’s briefly remind them here. In short, normally characters are encoded with 2 bytes. That gives us 65536 characters maximum. But there are more characters in the world.

So certain rare characters are encoded with 4 bytes, like 𝒳 (mathematical X) or 😄 (a smile).

Here are the unicode values to compare:

Character Unicode Bytes
a 0x0061 2
0x2248 2
𝒳 0x1d4b3 4
𝒴 0x1d4b4 4
😄 0x1f604 4

So characters like a and occupy 2 bytes, and those rare ones take 4.

The unicode is made in such a way that the 4-byte characters only have a meaning as a whole.

In the past JavaScript did not know about that, and many string methods still have problems. For instance, length thinks that here are two characters:

alert('😄'.length); // 2
alert('𝒳'.length); // 2

…But we can see that there’s only one, right? The point is that length treats 4 bytes as two 2-byte characters. That’s incorrect, because they must be considered only together (so-called “surrogate pair”).

Normally, regular expressions also treat “long characters” as two 2-byte ones.

That leads to odd results, for instance let’s try to find [𝒳𝒴] in the string 𝒳:

alert( '𝒳'.match(/[𝒳𝒴]/) ); // odd result (wrong match actually, "half-character")

The result is wrong, because by default the regexp engine does not understand surrogate pairs.

So, it thinks that [𝒳𝒴] are not two, but four characters:

  1. the left half of 𝒳 (1),
  2. the right half of 𝒳 (2),
  3. the left half of 𝒴 (3),
  4. the right half of 𝒴 (4).

We can list them like this:

for(let i=0; i<'𝒳𝒴'.length; i++) {
  alert('𝒳𝒴'.charCodeAt(i)); // 55349, 56499, 55349, 56500

So it finds only the “left half” of 𝒳.

In other words, the search works like '12'.match(/[1234]/): only 1 is returned.

The “u” flag

The /.../u flag fixes that.

It enables surrogate pairs in the regexp engine, so the result is correct:

alert( '𝒳'.match(/[𝒳𝒴]/u) ); // 𝒳

Let’s see one more example.

If we forget the u flag and occasionally use surrogate pairs, then we can get an error:

'𝒳'.match(/[𝒳-𝒴]/); // SyntaxError: invalid range in character class

Normally, regexps understand [a-z] as a "range of characters with codes between codes of a and z.

But without u flag, surrogate pairs are assumed to be a “pair of independant characters”, so [𝒳-𝒴] is like [<55349><56499>-<55349><56500>] (replaced each surrogate pair with code points). Now we can clearly see that the range 56499-55349 is unacceptable, as the left range border must be less than the right one.

Using the u flag makes it work right:

alert( '𝒴'.match(/[𝒳-𝒵]/u) ); // 𝒴
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