How many possible Unicode characters there are and why

How many?

The previous post showed how the number of Unicode characters has grown over time.

Unicode size by version

You’ll notice there was a big jump between versions 3.0 and 3.1. That will be important later on.

Unicode started out relative small then became much more ambitious. Are they going to run out of room? How many possible Unicode characters are there?

Short answer: There are 1,111,998 possible Unicode characters.

Longer answer: There are 17×216 – 2048 – 66 = 1,111,998 possible Unicode characters: seventeen 16-bit planes, with 2048 values reserved as surrogates, and 66 reserved as non-characters. More on this below.

Which ones?

Going one level of detail deeper, which numbers correspond to Unicode characters?

The hexadecimal numbers 0 through 10FFFF are potential Unicode characters, with exception of surrogates and non-characters.

Unicode is divided into 17 planes. The first two hexadecimal “digits” indicate the plane, and the last four indicate a value within the plane. The first plane is known as the BMP, the Basic Multilingual Plane. The rest are known as supplemental planes.

The surrogates are DC00 through DFFF and D800 through DBFF. The first range of 1024 surrogates are known as low surrogates, and the second rage of 1024 the high surrogates.

The non-characters are FDD0 through FDEF and the last two values in each plane: FFFE, FFFF, 1FFFE, 1FFFF, 2FFFE, 2FFFF, …, 10FFFE, 10FFFF. This is one range of 32 non-characters, plus 34 coming from the end of each plane, for a total of 66.

Why?

Why are there only 17 planes? And what are these mysterious surrogates and non-characters? What purpose do they serve?

The limitations of UTF-16 encoding explain why 17 planes and why surrogates. Non-characters require a different explanation.

UTF-16

This post mentioned at the top that the size of Unicode jumped between versions 3.0 and 3.1. Significantly, the size went from less than 216 to more than 216. Unicode broke out of the Basic Multilingual Plane.

Unicode needed a way to represent more than 216 characters using groups of 16 bits. The solution to this problem was UTF-16 encoding. With this encoding, the surrogate values listed above do not represent characters per se but are a kind of pointer to further values.

Sixteen supplemental planes would take 20 bits to describe, 4 to indicate the plane and 16 for the values within the plane. The idea was to use a high surrogate to represent the first 10 bits and a low surrogate to represent the last 10 bits. The values DC00 through DFFF and D800 through DBFF were unassigned at the time, so they were picked for surrogates.

In a little more detail, a character in one of the supplemental planes is represented by a hexadecimal number between 1 0000 and 10 FFFF. If we subtract off 1 0000 we get a number between 0000 and FFFFF, a 20-bit number. Take the first 10 bits and add them to D800 to get a high surrogate value. Take the last 10 bits and add them to DC00 to get a low surrogate value. This pair of surrogate values represents the value in one of the supplemental planes.

When you encounter a surrogate value, you don’t need any further context to tell what it is. You don’t need to look upstream for some indication of how the bits are to be interpreted. It cannot be a BMP character, and there’s no doubt whether it is the beginning or end of a pair of surrogate values since the high surrogates and low surrogates are in different ranges.

UTF-16 can only represent 17 planes, and the Unicode Consortium decided they would not assign values that cannot be represented in UTF-16. So that’s why there are 17 planes.

Non-characters

That leaves the non-characters. Why are a few values reserved to never be used for characters?

One use for non-characters is to return a null value as an error indicator, analogous to a NaN or non-a-number in floating point calculations. A program might return FFFF, for example, to indicate that it was unable to read a character.

Another use for special non-characters is to imply which encoding method is used. For reasons that are too complicated to get into here, computers do not always store the bytes within a word in the increasing order. In so called “little endian” order, lower order bits are stored before higher order bits. (“Big endian” and “little endian” are allusions to the two factions in Gulliver’s Travels that crack boiled eggs on their big end and little end respectively.)

The byte order mark FEFF is inserted at the beginning of a file or stream to imply byte ordering. If it is received in the order FEFF then the byte stream is inferred to be using the big endian convention. But if it is received in the order FFFE then little endian is inferred because FFFE cannot be a character.

The preceding paragraphs give a justification for at least two non-characters, FFFF and FFFE, but it’s not clear why 66 are reserved. There could be reasons for each plane would have its own FFFF and FFFE, which would account for 34 non-characters. I’m not clear on why FDD0 through FDEF are non-characters, though I vaguely remember there being some historical reason. In any case, people are free to use the non-characters however they see fit.

Related posts

Growth of Unicode over time

My previous post quoted Randall Munroe saying Unicode “started out just trying to unify a couple different character sets” and grew much more ambitious.

The first version of Unicode, published in 1991, had 7,191 characters. Now the latest version has 137,994 characters and so is about 19 times bigger. Here’s a plot of the number of characters in Unicode over time.

Number of Unicode characters over time

Here’s a slightly different plot where the horizontal axis is version number rather than time.

Number of Unicode characters by standard version number

There’s plenty of room left in Unicode. The maximum number of possible Unicode characters is 1,111,998 for reasons I get into here.

The hopeless task of the Unicode Consortium

Randall Munroe, author of xkcd, discussing Unicode on the Triangulation podcast:

I am endlessly delighted by the hopeless task that the Unicode Consortium has created for themselves. … They started out just trying to unify a couple different character sets. And before they quite realized what was happening, they were grappling with decisions at the heart of how we use language, no matter how hard they tried to create policies to avoid these problems. It’s just a fun example of how weird language is and how hard human communication is and how you really can’t really get around those problems. … These are really hard problems and I do not envy them.

Reminds me of Jeffrey Snover’s remark about problems vs dilemmas: problems can be solved, but dilemmas can only be managed. Unicode faces a host of dilemmas.

Regarding Munroe’s comment about Unicode starting out small and getting more ambitious, see the next post for a plot of the number of characters as a function of time and of version number.

Related posts

Projecting Unicode to ASCII

Sometimes you need to downgrade Unicode text to more restricted ASCII text. For example, while working on my previous post, I was surprised that there didn’t appear to be an asteroid named after Poincaré. There is one, but it was listed as Poincare in my list of asteroid names.

Python module

I used the Python module unidecode to convert names to ASCII before searching, and that fixed the problem. Here’s a small example showing how the code works.

    import unidecode

    for x in ["Poincaré", "Gödel"]:
        print(x, unidecode.unidecode(x))

This produces

    Poincaré Poincare
    Gödel Godel

Installing the unidecode module also installs a command line utility by the same name. So you could, for example, pipe text to that utility.

As someone pointed out on Hacker News, this isn’t so impressive for Western languages,

But if you need to project Arabic, Russian or Chinese, unidecode is close to black magic:

>>> from unidecode import unidecode
>>> unidecode("北亰")
'Bei Jing '

(Someone has said in the comments that 北亰 is a typo and should be 北京. I can’t say whether this is right, but I can say that unidecode transliterates both to “Bei Jing.”)

Projections

I titled this post “Projecting Unicode to ASCII” because this code is a projection in the mathematical sense. A projection is a function P such that for all inputs x,

PP(x) ) = P(x).

That is, applying the function twice does the same thing as applying the function once. The name comes from projection in the colloquial sense, such as projecting a three dimensional object onto a two dimensional plane. An equivalent term is to say P is idempotent. [1]

The unidecode function maps the full range of Unicode characters into the range 0x00 to 0x7F, and if you apply it to a character already in that range, the function leaves it unchanged. So the function is a projection, or you could say the function is idempotent.

Projection is such a simple condition that it hardly seems worth giving it a name. And yet it is extremely useful. A general principle in user interface to design is to make something a projection if the user expects it to be a projection. Users probably don’t have the vocabulary to say “I expected this to be a projection” but they’ll be frustrated if something is almost a projection but not quite.

For example, if software has a button to convert an image from color to grayscale, it would be surprising if (accidentally) clicking button a second time had any effect. It would be unexpected if it returned the original color image, and it would be even more unexpected if it did something else, such as keeping the image in grayscale but lowering the resolution.

Related posts

[1] The term “idempotent” may be used more generally than “projection,” the latter being more common in linear algebra. Some people may think of a projection as linear idempotent function. We’re not exactly doing linear algebra here, but people do think of portions of Unicode geometrically, speaking of “planes.”

Trademark symbol, LaTeX, and Unicode

Earlier this year I was a coauthor on a paper about the Cap Score™ test for male fertility from Androvia Life Sciences [1]. I just noticed today that when I added the publication to my CV, it caused some garbled text to appear in the PDF.

garbled LaTeX output

Here is the corresponding LaTeX source code.

LaTeX source

Fixing the LaTeX problem

There were two problems: the trademark symbol and the non-printing symbol denoted by a red underscore in the source file. The trademark was a non-ASCII character (Unicode U+2122) and the underscore represented a non-printing (U+00A0). At first I only noticed the trademark symbol, and I fixed it by including a LaTeX package to allow Unicode characters:

    \usepackage[utf8x]{inputenc}

An alternative fix, one that doesn’t require including a new package, would be to replace the trademark Unicode character with \texttrademark\. Note the trailing backslash. Without the backslash there would be no space after the trademark symbol. The problem with the unprintable character would remain, but the character could just be deleted.

Trademark and Unicode

I found out there are two Unicode code points render the trademark glyph, U+0099 and U+2122. The former is in the Latin 1 Supplement section and is officially a control character. The correct code point for the trademark symbol is the latter. Unicode files U+2122 under Letterlike Symbols and gives it the official name TRADE MARK SIGN.

Related posts

[1] Jay Schinfeld, Fady Sharara, Randy Morris, Gianpiero D. Palermo, Zev Rosenwaks, Eric Seaman, Steve Hirshberg, John Cook, Cristina Cardona, G. Charles Ostermeier, and Alexander J. Travis. Cap-Score™ Prospectively Predicts Probability of Pregnancy, Molecular Reproduction and Development. To appear.

Typesetting modal logic

Modal logic extends propositional logic with two new operators, □ (“box”) and ◇ (“diamond”). There are many interpretations of these two symbols, the most common being necessity and possibility respectively. That is, □p means the proposition p is necessary, and ◇p means that p is possible. Another interpretation is using the symbols to represent things a person knows to be true and things that may be true as far as that person knows.

There are also many axiom systems for inference concerning these operators. For example, some axiom systems include the rule

\Box p \rightarrow \Box \Box p

and some do not. If you interpret □ as saying a proposition is provable, this axiom says whatever is provable is provably provable, which makes sense. But if you take □ to be a statement about what an agent knows, you may not want to say that if an agent knows something, it knows that it knows it.

See the next post for an example of applying logic to security, a logic with lots of modal operators and axioms. But for now, we’ll focus on how to typeset the box and diamond operators.

LaTeX

In LaTeX, the most obvious commands would be \box and \diamond, but that doesn’t work. There is no \box command, though there is a \square command. And although there is a \diamond command, it produces a symbol much smaller than \square and so the two look odd together. The two operators are dual in the sense that

\begin{align*} \Box p &= \neg \Diamond \neg p \\ \Diamond p &= \neg \Box \neg p \end{align*}

and so they should have symbols of similar size. A better approach is to use \Box and \Diamond. Those were used in the displayed equations above.

Unicode

There are many box-like and diamond-like symbols in Unicode. It seems reasonable to use U+25A1 for box and U+25C7 for diamond. I don’t know of any more semantically appropriate characters. There are no Unicode characters with “modal” in their name, for example.

HTML

You can always insert Unicode characters into HTML by using &#x, followed by the hexadecimal value of the codepoint, followed by a semicolon. For example, I typed □ and ◇ to enter the box and diamond symbols above.

If you want to stick to HTML entities because they’re easier to remember, you’re mostly out of luck. There is no HTML entity for the box operator. There is an entity ◊ for “lozenge,” the typographical term for a diamond. This HTML entity corresponds to U+25CA and is smaller than U+25c7 recommended above. As discussed in the context of LaTeX, you want the box and diamond operators to have a similar size.

Related posts

Fraktur symbols in mathematics

When mathematicians run out of symbols, they turn to other alphabets. Most math symbols are Latin or Greek letters, but occasionally you’ll run into Russian or Hebrew letters.

Sometimes math uses a new font rather than a new alphabet, such as Fraktur. This is common in Lie groups when you want to associate related symbols to a Lie group and its Lie algebra. By convention a Lie group is denoted by an ordinary Latin letter and its associated Lie algebra is denoted by the same letter in Fraktur font.

lower case alphabet in Fraktur

LaTeX

To produce Fraktur letters in LaTeX, load the amssymb package and use the command \mathfrak{}.

Symbols such as \mathfrak{A} are math symbols and can only be used in math mode. They are not intended to be a substitute for setting text in Fraktur font. This is consistent with the semantic distinction in Unicode described below.

Unicode

The Unicode standard tries to distinguish the appearance of a symbol from its semantics, though there are compromises. For example, the Greek letter Ω has Unicode code point U+03A9 but the symbol Ω for electrical resistance in Ohms is U+2621 even though they are rendered the same [1].

The letters a through z, rendered in Fraktur font and used as mathematical symbols, have Unicode values U+1D51E through U+1D537. These values are in the “Supplementary Multilingual Plane” and do not commonly have font support [2].

The corresponding letters A through Z are encoded as U+1D504 through U+1D51C, though interestingly a few letters are missing. The code point U+1D506, which you’d expect to be Fraktur C, is reserved. The spots corresponding to H, I, and R are also reserved. Presumably these are reserved because they are not commonly used as mathematical symbols. However, the corresponding bold versions U+1D56C through U+ID585 have no such gaps [3].

Footnotes

[1] At least they usually are. A font designer could choose provide different glyphs for the two symbols. I used the same character for both because some I thought some readers might not see the Ohm symbol properly rendered.

[2] If you have the necessary fonts installed you should see the alphabet in Fraktur below:
𝔞 𝔟 𝔠 𝔡 𝔢 𝔣 𝔤 𝔥 𝔦 𝔧 𝔨 𝔩 𝔪 𝔫 𝔬 𝔭 𝔮 𝔯 𝔰 𝔱 𝔲 𝔳 𝔴 𝔵 𝔶 𝔷

I can see these symbols from my desktop and from my iPhone, but not from my Android tablet. Same with the symbols below.

[3] Here are the bold upper case and lower case Fraktur letters in Unicode:
𝕬 𝕭 𝕮 𝕯 𝕰 𝕱 𝕲 𝕳 𝕴 𝕵 𝕶 𝕷 𝕸 𝕹 𝕺 𝕻 𝕼 𝕽 𝕾 𝕿 𝖀 𝖁 𝖂 𝖃 𝖄 𝖅
𝖆 𝖇 𝖈 𝖉 𝖊 𝖋 𝖌 𝖍 𝖎 𝖏 𝖐 𝖑 𝖒 𝖓 𝖔 𝖕 𝖖 𝖗 𝖘 𝖙 𝖚 𝖛 𝖜 𝖝 𝖞 𝖟

Why don’t you simply use XeTeX?

From an FAQ post I wrote a few years ago:

This may seem like an odd question, but it’s actually one I get very often. On my TeXtip twitter account, I include tips on how to create non-English characters such as using \AA to produce Å. Every time someone will ask “Why not use XeTeX and just enter these characters?”

If you can “just enter” non-English characters, then you don’t need a tip. But a lot of people either don’t know how to do this or don’t have a convenient way to do so. Most English speakers only need to type foreign characters occasionally, and will find it easier, for example, to type \AA or \ss than to learn how to produce Å or ß from a keyboard. If you frequently need to enter Unicode characters, and know how to do so, then XeTeX is great.

One does not simply type Unicode characters.

More Unicode posts

Graphemes

Here’s something amusing I ran across in the glossary of Programming Perl:

grapheme A graphene is an allotrope of carbon arranged in a hexagonal crystal lattice one atom thick. Grapheme, or more fully, a grapheme cluster string is a single user-visible character, which in turn may be several characters (codepoints) long. For example … a “ȫ” is a single grapheme but one, two, or even three characters, depending on normalization.

In case the character ȫ doesn’t display correctly for you, here it is:

Unicode character U_022B

First, graphene has little to do with grapheme, but it’s geeky fun to include it anyway. (Both are related to writing. A grapheme has to do with how characters are written, and the word graphene comes from graphite, the “lead” in pencils. The origin of grapheme has nothing to do with graphene but was an analogy to phoneme.)

Second, the example shows how complicated the details of Unicode can get. The Perl code below expands on the details of the comment about ways to represent ȫ.

This demonstrates that the character . in regular expressions matches any single character, but \X matches any single grapheme. (Well, almost. The character . usually matches any character except a newline, though this can be modified via optional switches. But \X matches any grapheme including newline characters.)

   
# U+0226, o with diaeresis and macron 
my $a = "\x{22B}"; 

# U+00F6 U+0304, (o with diaeresis) + macron 
my $b = "\x{F6}\x{304}";    
     
# o U+0308 U+0304, o + diaeresis + macron   
my $c = "o\x{308}\x{304}"; 

my @versions = ($a, $b, $c);

# All versions display the same.
say @versions;

# The versions have length 1, 2, and 3.
# Only $a contains one character and so matches .
say map {length $_ if /^.$/} @versions;

# All versions consist of one grapheme.
say map {length $_ if /^\X$/} @versions;

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