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9-regular-expressions/20-regexp-unicode/article.md

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+# Unicode: flag "u"
+
+The unicode flag `/.../u` enables the correct support of surrogate pairs.
+
+Surrogate pairs are explained in the chapter <info:string>.
+
+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:
+
+```js run
+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 `pattern:[𝒳𝒴]` in the string `subject:𝒳`:
+
+```js run
+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:
+
+```js run
+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:
+
+```js run
+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:
+
+```js run
+'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:
+
+```js run
+alert( '𝒴'.match(/[𝒳-𝒵]/u) ); // 𝒴
+```