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10-regular-expressions-javascript/06-regexp-unicode/article.md

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+# The unicode flag
+
+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
+```
+
+The result would be wrong, because by default the regexp engine does not understand surrogate pairs. It thinks that `[𝒳𝒴]` are not two, but four characters: the left half of `𝒳` `(1)`, the right half of `𝒳` `(2)`, the left half of `𝒴` `(3)`, the right half of `𝒴` `(4)`.
+
+So it finds the left half of `𝒳` in the string `𝒳`, not the whole symbol.
+
+In other words, the search works like `'12'.match(/[1234]/)` -- the `1` is returned (left half of `𝒳`).
+
+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
+```
+
+There's an error that may happen if we forget the flag:
+
+```js run
+'𝒳'.match(/[𝒳-𝒴]/); // SyntaxError: invalid range in character class
+```
+
+Here the regexp `[𝒳-𝒴]` is treated as `[12-34]` (where `2` is the right part of `𝒳` and `3` is the left part of `𝒴`), and the range between two halves `2` and `3` is unacceptable.
+
+Using the flag would make it work right:
+
+```js run
+alert( '𝒴'.match(/[𝒳-𝒵]/u) ); // 𝒴
+```
+
+To finalize, let's note that if we do not deal with surrogate pairs, then the flag does nothing for us. But in the modern world we often meet them.