Ask HN: What are your Unicode woes?
4 Rendello 5 6/14/2025, 2:48:43 PM
I've always worked with text, but I only started digging deep into understanding Unicode this year.
What do HN people have to say about Unicode and UTF-{8,16,32}? Are there parts you've never really understood? Have you had unexpected bugs due to misunderstood properties of text?
The normalization forms are explained, in order of approachability (imo), in this random Youtube video, the Unicode Annex #15, and the Unicode Core Spec:
https://www.youtube.com/watch?v=ttLD4DiMpiQ
https://unicode.org/reports/tr15/
https://www.unicode.org/versions/Unicode16.0.0/core-spec/cha...
Turns out, sometimes changing case changes not only the number of bytes (in UTF-8), but the number of encoded characters! This led to my post "UTF-8 characters that behave oddly when the case is changed" [1], which inspired a lot of conversation that taught me a lot. After that, I started reading Unicode documentation in earnest, and building up an idea of what a new tool should show. I'm trying to make clear things I didn't (and sometimes still don't) understand, so I'd love to know what causes pains in the wild / gaps in people's understanding.
1. https://news.ycombinator.com/item?id=42014045
> *2.2.3 Characters, Not Glyphs*
> The Unicode Standard draws a distinction between characters and glyphs. Characters are the abstract representations of the smallest components of written language that have semantic value. They represent primarily, but not exclusively, the letters, punctuation, and other signs that constitute natural language text and technical notation. [...] Letters in different scripts, even when they correspond either semantically or graphically, are represented in Unicode by distinct characters.
> Characters are represented by code points that reside only in a memory representation, as strings in memory, on disk, or in data transmission. The Unicode Standard deals only with character codes.
> *2.4 Code Points and Characters*
> The range of integers used to code the abstract characters is called the codespace. A particular integer in this set is called a code point. When an abstract character is mapped or assigned to a particular code point in the codespace, it is then referred to as an encoded character.
> *2.5 Encoding Forms*
This deals with UTF-{8,16,32}, which is a tricky bit and tripped me up for a long time. If the document is too dense here, there's a lot of supplementary material online explaining the different forms, I'll link a Tom Scott video explaining UTF-8.
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The long and short of it is: the atomic unit of Unicode is the character, or encoded character, which is a value that has been associated with a code point, which is an integer usually represented in hex for as U+XXXX. Unicode doesn't deal with glyphs or graphical representations, just characters and their properties (eg. what is the character name? what should this character do when uppercased?). As you probably know, many characters can combine with others to form grapheme clusters, which may look like a single (abstract) character, but underneath consist of multiple (encoded) characters. Every character is associated with an integer index (a codepoint), and those integers can be represented in three formats (this sort of happened by accident): UTF-32 (just represent the integer directly), UTF-16 (was originally supposed to represent the integer directly, but there were too many and it got extended), and UTF-8 (which has different byte lengths to encode different characters efficiently).
[spec] https://www.unicode.org/versions/Unicode16.0.0/core-spec/
[2.2.3] https://www.unicode.org/versions/Unicode16.0.0/core-spec/cha...
[2.4] https://www.unicode.org/versions/Unicode16.0.0/core-spec/cha...
[2.5] https://www.unicode.org/versions/Unicode16.0.0/core-spec/cha...
[Tom Scott UTF-8] https://www.youtube.com/watch?v=MijmeoH9LT4