Summary of our discussion on XML character encoding and decodingand inte

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

This has been an outstanding discussion. Thanks to all those who participated.

Below I have summarized most of our discussion.
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Table of Contents
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1. Excellent quotes
2. Lots of character conversions taking place inside our computers and on the Web
3. Well-formedness error when encoding="..." does not match actual character encoding
4. How to generate an encoding well-formedness error
5. If everyone used UTF-8 would that be best for everyone?
6. Interoperability of XML (i.e., Character Encoding Interoperability)

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1. Excellent quotes
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    ... around 1972 ... "Why is character coding so complicated?" ... it hasn't 
    become any simpler in the intervening 40 years.

    To be able to track end-to-end the path of conversions and validate that 
    your application from authoring through to storage through to search 
    and retrieval is completely correct is amazingly difficult ... it's a skill 
    far too few programmers have, or even recognize that they do not have.

    We can't really tell what's going on without access to your entire tool chain.  

    It's possible that your editor changed the character encoding of your text 
    when you changed the XML declaration (emacs does this)!

    It's unlikely that the encoding of the characters in this email is byte-identical 
    with the files you created.

    ... my preferred solution is to stick to a single encoding everywhere ...

    ... I vote for UTF-8 ... 

    ... make sure *every single link in the chain* uses that encoding.

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2. Lots of character conversions taking place inside our computers and on the Web
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All of the characters in the following XML are encoded in iso-8859-1:

<?xml version="1.0" encoding="iso-8859-1"?>
<Name>López</Name>

Now consider this problem: 

    Suppose we search the XML for the string López, 
    where the characters in López are encoded in UTF-8. 
    Will the search application find a match?

I did the search and it found a match.

How did it find a match?

The underlying byte sequence for the iso-8859-1 López is: 4C F3 70 65 7A (one byte -- F3 -- is used to encode ó).

The underlying byte sequence for the UTF-8 López is: 4C C3 B3 70 65 7A (two bytes -- C3 B3 -- are used to encode ó).

The search application cannot be doing a byte-for-byte match, else it would find no match.

The answer is that the search application either converted the XML to UTF-8 or converted the search string (López) to iso-8859-1.

Inside our computers, inside our software applications, and on the Web there is a whole lot of character encoding conversions occurring ... transparently ... without our knowing.

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3. Well-formedness error when encoding="..." does not match actual character encoding
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Create an XML document and encode all the characters in it using UTF-8: 

    <Name>López</Name>

Add an XML declaration and specify that the encoding is iso-8859-1:

    <?xml version="1.0" encoding="iso-8859-1"?>
    <Name>López</Name>

There is a mismatch between what encoding="..." says and the actual encoding of the characters in the document.

Check the XML document for well-formedness and you will NOT get an error.

Next, create the same XML document but encode all the characters using iso-8859-1: 

    <Name>López</Name>

Add an XML declaration and specify the encoding is UTF-8:

    <?xml version="1.0" encoding="utf-8"?>
    <Name>López</Name>

Again there is a mismatch between what encoding="..." says and the actual encoding of the characters in the document.

But this time when you check the XML document for well-formedness you WILL get an error.

Here's why.

In UTF-8 the ó symbol is encoded using these two bytes: C3 B3

In iso-8859-1, C3 and B3 represent two perfectly fine characters, so the UTF-8 encoded XML is a fine encoding="iso-8859-1" document.

In iso-8859-1 the ó symbol is encoded using one byte: F3

F3 is not a legal UTF-8 byte, so the iso-8859-1 encoded XML fails as an encoding="UTF-8" document.

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4. How to generate an encoding well-formedness error
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George Cristian Bina from oXygen XML gave the scoop on how things work inside oXygen.

1. Create an XML document and encode all the characters in it using iso-8859-1:

    <?xml version="1.0" encoding="iso-8859-1"?>
    <Name>López</Name>

2. Using a hex editor, change encoding="iso-8859-1" to encoding="utf-8":

    <?xml version="1.0" encoding="utf-8"?>
    <Name>López</Name>

3. Drag and drop the file into oXygen.

4. oXygen will generate an encoding exception: 

    Cannot open the specified file. Got a character 
    encoding exception [snip]

George described the encoding conversions that occur inside oXygen behind-the-scenes:

    If you have an iso-8859-1 encoded XML file loaded into oXygen 
    and change encoding="iso-8859-1" to encoding="utf-8" then 
    oXygen will automatically change the encoding of every character 
    in the document to UTF-8.

George also made this important comment:

    Please note that the encoding is important only when the file is loaded 
    and saved. When the file is loaded the bytes are converted to characters 
    and then the application works only with characters. When the file is 
    saved then those characters need to be converted to bytes and the 
    encoding used will be determined from the XML header with a default to 
    UTF-8 if no encoding can be detected.

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5. If everyone used UTF-8 would that be best for everyone?
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There are a multiplicity of character encodings and a huge number of character encoding conversions taking place behind-the-scene. If everyone used UTF-8 then there would be no need to convert encodings.

Suppose every application, every IDE, every text editor, and every system worldwide used one character encoding, UTF-8. 

Would that be a good thing?

The trouble with that is that UTF-8 makes larger files than UTF-16 for great numbers of people who use ideographic scripts such as Chinese. The
real choice for them is between 16 and 32.

UTF-16 is also somewhat harder to process in some older programming languages, most notably C and C++, where a zero-valued byte (NUL, as
opposed to the zero-valued machine address, NULL) is used as a string terminator.

So UTF-8 isn't a universal solution.

There isn't a single solution today that's best for everyone.

See this excellent discussion on StackOverflow titled, Should UTF-16 be Considered Harmful?

http://programmers.stackexchange.com/questions/102205/should-utf-16-be-considered-harmful

In that discussion one person wrote:

    After long research and discussions, the development 
    conventions at my company ban using UTF-16 anywhere 
    except OS API calls ...

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6. Interoperability of XML (i.e., Character Encoding Interoperability)
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Remember not long ago you would visit a web page and see strange characters like this:

    <Hal>“Good morning, Daveâ€</Hal>

You don't see that much anymore. 

Why?

The answer is this:

    Interoperability is getting better.

In the context of character encoding and decoding, what does that mean?

Interoperability means that you and I interpret (decode) the bytes in the same way.

Example: I create an XML file, encode all the characters in it using UTF-8, and send the XML file to you. 

Here is a graphical depiction (i.e., glyphs) of the bytes that I send to you:

    <Name>López</Name>

You receive my XML document and interpret the bytes as iso-8859-1. 

In UTF-8 the ó symbol is a graphical depiction of the "LATIN SMALL LETTER O WITH ACUTE" character and it is encoded using these two bytes: C3 B3

But in iso-8859-1, the two bytes C3 B3 is the encoding of two characters:

     C3 is the encoding of the à character
     B3 is the encoding of the ³ character

Thus you interpret my XML as:

    <Name>López</Name>

We are interpreting the same XML (i.e., the same set of bytes) differently.

Interoperability has failed.

So when we say: 

    Interoperability is getting better.

we mean that the number of incidences of senders and receivers interpreting the same bytes differently is decreasing.  

Let's revisit our first example, you go to a web site and see this: 

     <Hal>“Good morning, Daveâ€</Hal>

Here's how that happened:

I use Microsoft Word (character set, Windows-1252) to create a web page containing this XML document:

    <Hal>“Good morning, Dave”</Hal>

Notice that I wrapped the greeting in Microsoft smart quotes. 

You visit my web page.

Suppose your browser is set to interpret all web pages as iso-8859-15.

In Windows-1252 the left smart quote is hex: 93

In Windows-1252 the right smart quote is hex: 84

In iso-8859-15 there are no characters assigned to either hex 93 or hex 84.  

So your browser replaces the left smart quote (hex 93) with hex E2 (â) followed by hex A4 (€) followed by hex BD (œ).

And your browser replaces the right smart quote (hex 84) with hex E2 (â) followed by hex A4 (€). 

The result is that you see this on your browser screen:

    <Hal>“Good morning, Daveâ€</Hal>