89 lines
3 KiB
Markdown
89 lines
3 KiB
Markdown
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# Unicode: flag "u"
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The unicode flag `/.../u` enables the correct support of surrogate pairs.
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Surrogate pairs are explained in the chapter <info:string>.
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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.
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So certain rare characters are encoded with 4 bytes, like `𝒳` (mathematical X) or `😄` (a smile).
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Here are the unicode values to compare:
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| Character | Unicode | Bytes |
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|------------|---------|--------|
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| `a` | 0x0061 | 2 |
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| `≈` | 0x2248 | 2 |
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|`𝒳`| 0x1d4b3 | 4 |
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|`𝒴`| 0x1d4b4 | 4 |
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|`😄`| 0x1f604 | 4 |
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So characters like `a` and `≈` occupy 2 bytes, and those rare ones take 4.
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The unicode is made in such a way that the 4-byte characters only have a meaning as a whole.
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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:
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```js run
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alert('😄'.length); // 2
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alert('𝒳'.length); // 2
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```
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...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").
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Normally, regular expressions also treat "long characters" as two 2-byte ones.
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That leads to odd results, for instance let's try to find `pattern:[𝒳𝒴]` in the string `subject:𝒳`:
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```js run
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alert( '𝒳'.match(/[𝒳𝒴]/) ); // odd result (wrong match actually, "half-character")
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```
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The result is wrong, because by default the regexp engine does not understand surrogate pairs.
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So, it thinks that `[𝒳𝒴]` are not two, but four characters:
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1. the left half of `𝒳` `(1)`,
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2. the right half of `𝒳` `(2)`,
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3. the left half of `𝒴` `(3)`,
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4. the right half of `𝒴` `(4)`.
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We can list them like this:
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```js run
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for(let i=0; i<'𝒳𝒴'.length; i++) {
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alert('𝒳𝒴'.charCodeAt(i)); // 55349, 56499, 55349, 56500
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};
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```
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So it finds only the "left half" of `𝒳`.
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In other words, the search works like `'12'.match(/[1234]/)`: only `1` is returned.
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## The "u" flag
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The `/.../u` flag fixes that.
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It enables surrogate pairs in the regexp engine, so the result is correct:
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```js run
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alert( '𝒳'.match(/[𝒳𝒴]/u) ); // 𝒳
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```
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Let's see one more example.
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If we forget the `u` flag and occasionally use surrogate pairs, then we can get an error:
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```js run
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'𝒳'.match(/[𝒳-𝒴]/); // SyntaxError: invalid range in character class
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```
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Normally, regexps understand `[a-z]` as a "range of characters with codes between codes of `a` and `z`.
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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.
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Using the `u` flag makes it work right:
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```js run
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alert( '𝒴'.match(/[𝒳-𝒵]/u) ); // 𝒴
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```
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