XML Reference Guide

Mar 14, 2003

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Character Sets and Encodings

In the beginning, there was ASCII. And it was good. ASCII is a 7-bit system for defining characters a computer should display. After all, when you get right down to it, a computer only understands numbers -- and binary numbers, at that.

So when I want to the computer to display, say an f, I can tell it to use ASCII character number 102, and the computer knows what I mean. It chooses the f charcater from the ASCII character set. That set, with the lowercase letters and special characters omitted for simplicity, looks something like this:

A	B	C	D	E	F	G	H	I	J	K	L	M
65	66	67	68	69	70	71	72	73	74	75	76	77

N	O	P	Q	R	S	T	U	V	W	X	Y	Z
78	79	80	81	82	83	84	85	86	87	88	89	90

So when I want to display

FOX OLYMPICS

The computer actually sees

70 79 88 32 79 76 89 77 80 73 67 83

But I could go ahead and create a different character set of, say

Ж	З	ٸ	Θ	إ	س	г	ئ	ش	В	Т	І	Ї
65	66	67	68	69	70	71	72	73	74	75	76	77

И	в	פ	ל	Ι	ٸ	ף	х	א	ۓ	ل	م	ن
78	79	80	81	82	83	84	85	86	87	88	89	90

so the same display instructions to the computer for the new characters actually tells it to display

سвل вІمЇפشٸٸ

Now, aside from amusing ourselves by making up alphabets, why do we care about this?

We care about it beause the ASCII character set only has space for 128 characters. That means that anybody speaking a language with non-ASCII characters, such as the ä or é, is out of luck in the ASCII way of doing things.

That makes things tough for a supposedly universal format such as XML.

So what's a poor data architect to do? Give up on reaching the billions of people whose native languages use non-ASCII characters? Of course not. Not in today's global economy. You could simply specify a different character set for a document in another language, but even if that were practical for exchanging data files -- which it's not -- it wouldn't work because some languages, such as Chinese, have not hundreds, but tens of thousands of characters.

Enter Unicode. Unicode is a character set that is actually 16 bits, or 65,536 characters instead of ASCII's 7 bit, 128 character set. About 50,000 of those characters have been defined, including the typical unaccented Latin alphabet, accented characters, tens of thousands of Asian ideograph characters, and other languages such as Hebrew, Russian, and Arabic.

So everybody's happy and that takes care of the problem, right?

Well, not quite. Chances are, you're not going to need 65,000+ characters in your document. In many cases, you're going to be using a much smaller "slice" of the character set pie. In fact, you can usually fit all of your characters into an 8 bit, 256 character set, giving you two advantages. First, most text editors will be able to read your document, and second, it'll only be half the size of the 16 bit version. So how do these "slices" work?

Say, for example, I were writing a document in Russian. I'd need the Cyrillic aphabet for that. In Unicode, the Cyrillic alphabet takes up the slots between 1024 and 1273. But notice, that's only 249 slots, so if I started with a lower number, such as 1 instead of 1024, I could fit it all into a 8 bit character subset. So the Ж character might be number 182 instead of number 1046.

These slices, or subsets, are called character encodings. For example, the Cyrillic alphabet uses an encoding called ISO-8859-5. In English speaking countries, we typically use the ISO-8859-1 encoding.

All of this matters in XML because we can use virtually any encoding we want, so long as we specify it on the document. For example:

<?xml version="1.0" encoding="ISO-8859-1">
<candy>
    <item>
        <name>Chocolate bar</name>
        <calories>250</calories>
    </item>
</candy>

XML processors are required to understand the UTF-8 and UTF-16 encodings. UTF-16 is, of course, the complete set of Unicode characters. UTF-8 is an 8 bit version of Unicode in which some characters are represented by up to three bytes rather than the traditional one byte. UTF-8 is the default encoding for XML, so if you leave out that optional encoding attribute, UTF-8 is assumed.

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A	B	C	D	E	F	G	H	I	J	K	L	M
65	66	67	68	69	70	71	72	73	74	75	76	77

N	O	P	Q	R	S	T	U	V	W	X	Y	Z
78	79	80	81	82	83	84	85	86	87	88	89	90

Ж	З	ٸ	Θ	إ	س	г	ئ	ش	В	Т	І	Ї
65	66	67	68	69	70	71	72	73	74	75	76	77

И	в	פ	ל	Ι	ٸ	ף	х	א	ۓ	ل	م	ن
78	79	80	81	82	83	84	85	86	87	88	89	90

A	B	C	D	E	F	G	H	I	J	K	L	M
65	66	67	68	69	70	71	72	73	74	75	76	77

N	O	P	Q	R	S	T	U	V	W	X	Y	Z
78	79	80	81	82	83	84	85	86	87	88	89	90

Ж	З	ٸ	Θ	إ	س	г	ئ	ش	В	Т	І	Ї
65	66	67	68	69	70	71	72	73	74	75	76	77

И	в	פ	ל	Ι	ٸ	ף	х	א	ۓ	ل	م	ن
78	79	80	81	82	83	84	85	86	87	88	89	90

A	B	C	D	E	F	G	H	I	J	K	L	M
65	66	67	68	69	70	71	72	73	74	75	76	77

N	O	P	Q	R	S	T	U	V	W	X	Y	Z
78	79	80	81	82	83	84	85	86	87	88	89	90

Ж	З	ٸ	Θ	إ	س	г	ئ	ش	В	Т	І	Ї
65	66	67	68	69	70	71	72	73	74	75	76	77

И	в	פ	ל	Ι	ٸ	ף	х	א	ۓ	ل	م	ن
78	79	80	81	82	83	84	85	86	87	88	89	90