UTF-8

Character encoding and character sets, in truth, are not that difficult to understand. But if you don't understand them, you are going to be caught by surprise by some of HTML Purifier's behavior, namely the fact that it operates UTF-8 or the limitations of the character encoding transformations it does. This document will walk you through determining the encoding of your system and how you should handle this information. It will stay away from excessive discussion on the internals of character encoding, but offer the information in asides that can easily be skipped.

Asides

Text in this formatting is an aside, interesting tidbits for the curious but not strictly necessary material to do the tutorial. If you read this text, you'll come out with a greater understanding of the underlying issues.

Finding the real encoding

In the beginning, there was ASCII, and things were simple. But they weren't good, for no one could write in Cryllic or Thai. So there exploded a proliferation of character encodings to remedy the problem by extending the characters ASCII could express. This ridiculously simplified version of the history of character encodings shows us that there are now many character encodings floating around.

A character encoding tells the computer how to interpret raw zeroes and ones into real characters. It usually does this by pairing numbers with characters.

There are many different types of character encodings floating around, but the ones we deal most frequently with are ASCII, 8-bit encodings, and Unicode-based encodings.

• ASCII is a 7-bit encoding based on the English alphabet.
• 8-bit encodings are extensions to ASCII that add a potpourri of useful, non-standard characters like é and æ. They can only add 127 characters, so usually only support one script at a time. When you see a page on the web, chances are it's encoded in one of these encodings.
• Unicode-based encodings implement the Unicode standard and include UTF-8, UCS-2 and UTF-16. They go beyond 8-bits (the first two are variable length, while the second one uses 16-bits), and support almost every language in the world. UTF-8 is gaining traction as the dominant international encoding of the web.

The first step of our journey is to find out what the encoding of your website is. The most reliable way is to ask your browser:

Mozilla Firefox

Tools > Page Info: Encoding

Internet Explorer

View > Encoding: bulleted item is unofficial name

Internet Explorer won't give you the mime (i.e. useful/real) name of the character encoding, so you'll have to look it up using their description. Some common ones:

Internet Explorer does not recognize some of the more obscure character encodings, and having to lookup the real names with a table is a pain, so I recommend using Mozilla Firefox to find out your character encoding.

Finding the embedded encoding

At this point, you may be asking, "Didn't we already find out our encoding?" Well, as it turns out, there are multiple places where a web developer can specify a character encoding, and one such place is in a META tag:

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />

You'll find this in the HEAD section of an HTML document. The text to the right of charset= is the "claimed" encoding: the HTML claims to be this encoding, but whether or not this is actually the case depends on other factors. For now, take note if your META tag claims that either:

1. The character encoding is the same as the one reported by the browser,
2. The character encoding is different from the browser's, or
3. There is no META tag at all! (horror, horror!)

Fixing the encoding

If your META encoding and your real encoding match, savvy! You can skip this section. If they don't...

No embedded encoding

If this is the case, you'll want to add in the appropriate META tag to your website. It's as simple as copy-pasting the code snippet above and replacing UTF-8 with whatever is the mime name of your real encoding.

For all those skeptics out there, there is a very good reason why the character encoding should be explicitly stated. When the browser isn't told what the character encoding of a text is, it has to guess: and sometimes the guess is wrong. Hackers can manipulate this guess in order to slip XSS pass filters and then fool the browser into executing it as active code. A great example of this is the Google UTF-7 exploit.

You might be able to get away with not specifying a character encoding with the META tag as long as your webserver sends the right Content-Type header, but why risk it? Besides, if the user downloads the HTML file, there is no longer any webserver to define the character encoding.

Embedded encoding disagrees

This is an extremely common mistake: another source is telling the browser what the character encoding is and is overriding the embedded encoding. This source usually is the Content-Type HTTP header that the webserver (i.e. Apache) sends. A usual Content-Type header sent with a page might look like this:

Content-Type: text/html; charset=ISO-8859-1

Notice how there is a charset parameter: this is the webserver's way of telling a browser what the character encoding is, much like the META tags we touched upon previously.

In fact, the META tag is designed as a substitute for the HTTP header for contexts where sending headers is impossible (such as locally stored files without a webserver). Thus the name http-equiv (HTTP equivalent).

There are two ways to go about fixing this: changing the META tag to match the HTTP header, or changing the HTTP header to match the META tag. How do we know which to do? It depends on the website's content: after all, headers and tags are only ways of describing the actual characters on the web page.

If your website:

...only uses ASCII characters,

Either way is fine, but I recommend switching both to UTF-8 (more on this later).

...uses special characters, and they display properly,

Change the embedded encoding to the server encoding.

...uses special characters, but users often complain that they come out garbled,

Change the server encoding to the embedded encoding.

Changing a META tag is easy: just swap out the old encoding for the new. Changing the server (HTTP header) encoding, however, is slightly more difficult.

Changing the server encoding

PHP header() function

The simplest way to handle this problem is to send the encoding yourself, via your programming language. Since you're using HTML Purifier, I'll assume PHP, although it's not too difficult to do similar things in other languages. The appropriate code is:

header('Content-Type:text/html; charset=UTF-8');

...replacing UTF-8 with whatever your embedded encoding is. This code must come before any output, so be careful about stray whitespace in your application.

PHP ini directive

PHP also has a neat little ini directive that can save you a header call: default_charset. Using this code:

ini_set('default_charset', 'UTF-8');

...will also do the trick. If PHP is running as an Apache module (and not as FastCGI, consult phpinfo() for details), you can even use htaccess do apply this property globally:

php_value default_charset "UTF-8"

As with all INI directives, this can also go in your php.ini file. Some hosting providers allow you to customize your own php.ini file, ask your support for details. Use:

default_charset = "utf-8"

Non-PHP

You may, for whatever reason, may need to set the character encoding on non-PHP files, usually plain ol' HTML files. Doing this is more of a hit-or-miss process: depending on the software being used as a webserver and the configuration of that software, certain techniques may work, or may not work.

.htaccess

On Apache, you can use an .htaccess file to change the character encoding. I'll defer to W3C for the in-depth explanation, but it boils down to creating a file named .htaccess with the contents:

AddCharset UTF-8 .html

Where UTF-8 is replaced with the character encoding you want to use and .html is a file extension that this will be applied to. This character encoding will then be set for any file directly in or in the subdirectories of directory you place this file in.

If you're feeling particularly courageous, you can use:

AddDefaultCharset UTF-8

...which changes the character set Apache adds to any document that doesn't have any Content-Type parameters. This directive, which the default configuration file sets to iso-8859-1 for security reasons, is probably why your headers mismatch with the META tag. If you would prefer Apache not to be butting in on your character encodings, you can tell it not to send anything at all:

AddDefaultCharset Off

...making your META tags the sole source of character encoding information. In these cases, it is especially important to make sure you have valid META tags on your pages and all the text before them is ASCII.

These directives can also be placed in httpd.conf file for Apache, but in most shared hosting situations you won't be able to edit this file.

File extensions

If you're not allowed to use .htaccess files, you can often piggy-back off of Apache's default AddCharset declarations to get your files in the proper extension. Here are Apache's default character set declarations:

So, for example, a file named page.utf8.html or page.html.utf8 will probably be sent with the UTF-8 charset attached, the difference being that if there is an AddCharset charset .html declaration, it will override the .utf8 extension in page.utf8.html (precedence moves from right to left). By default, Apache has no such declaration.

Microsoft IIS

If anyone can contribute information on how to configure Microsoft IIS to change character encodings, I'd be grateful.

XML

META tags are the most common source of embedded encodings, but they can also come from somewhere else: XML processing instructions. They look like:

<?xml version="1.0" encoding="UTF-8"?>

...and are most often found in XML documents (including XHTML).

For XHTML, this processing instruction theoretically overrides the META tag. In reality, this happens only when the XHTML is actually served as legit XML and not HTML, which is almost always never due to Internet Explorer's lack of support for application/xhtml+xml (even though doing so is often argued to be good practice).

For XML, however, this processing instruction is extremely important. Since most webservers are not configured to send charsets for .xml files, this is the only thing a parser has to go on. Furthermore, the default for XML files is UTF-8, which often butts heads with more common ISO-8859-1 encoding (you see this in garbled RSS feeds).

In short, if you use XHTML and have gone through the trouble of adding the XML header, be sure to make sure it jives with your META tags and HTTP headers.

Inside the process

This section is not required reading, but may answer some of your questions on what's going on in all this character encoding hocus pocus. If you're interested in moving on to the next phase, skip this section.

A logical question that follows all of our wheeling and dealing with multiple sources of character encodings is "Why are there so many options?" To answer this question, we have to turn back our definition of character encodings: they allow a program to interpret bytes into human-readable characters.

Thus, a chicken-egg problem: a character encoding is necessary to interpret the text of a document. A META tag is in the text of a document. The META tag gives the character encoding. How can we determine the contents of a META tag, inside the text, if we don't know it's character encoding? And how do we figure out the character encoding, if we don't know the contents of the META tag?

Fortunantely for us, the characters we need to write the META are in ASCII, which is pretty much universal over every character encoding that is in common use today. So, all the web-browser has to do is parse all the way down until it gets to the Content-Type tag, extract the character encoding tag, then re-parse the document according to this new information.

Obviously this is complicated, so browsers prefer the simpler and more efficient solution: get the character encoding from a somewhere other than the document itself, i.e. the HTTP headers, much to the chagrin of HTML authors who can't set these headers.

Why UTF-8?

So, you've gone through all the trouble of ensuring that your server and embedded characters all line up properly and are present. Good job: at this point, you could quit and rest easy knowing that your pages are not vulnerable to character encoding style XSS attacks. However, just as having a character encoding is better than having no character encoding at all, having UTF-8 as your character encoding is better than having some other random character encoding, and the next step is to convert to UTF-8. But why?

Internationalization

Many software projects, at one point or another, suddenly realize that they should be supporting more than one language. Even regular usage in one language sometimes requires the occasional special character that, without surprise, is not available in your character set. Sometimes developers get around this by adding support for multiple encodings: when using Chinese, use Big5, when using Japanese, use Shift-JIS, when using Greek, etc. Other times, they use character entities with great zeal.

UTF-8, however, obviates the need for any of these complicated measures. After getting the system to use UTF-8 and adjusting for sources that are outside the hand of the browser (more on this later), UTF-8 just works. You can use it for any language, even many languages at once, you don't have to worry about managing multiple encodings, you don't have to use those user-unfriendly entities.

User-friendly

Websites encoded in Latin-1 (ISO-8859-1) which ocassionally need a special character outside of their scope often will use a character entity to achieve the desired effect. For instance, θ can be written θ, regardless of the character encoding's support of Greek letters.

This works nicely for limited use of special characters, but say you wanted this sentence of Chinese text: 激光, 這兩個字是甚麼意思. The entity-ized version would look like this:

激光, 這兩個字是甚麼意思

Extremely inconvenient for those of us who actually know what character entities are, totally unintelligible to poor users who don't! Even the slightly more user-friendly, "intelligible" character entities like θ will leave users who are uninterested in learning HTML scratching their heads. On the other hand, if they see θ in an edit box, they'll know that it's a special character, and treat it accordingly, even if they don't know how to write that character themselves.

Wikipedia is a great case study for an application that originally used ISO-8859-1 but switched to UTF-8 when it became far to cumbersome to support foreign languages. Bots will now actually go through articles and convert character entities to their corresponding real characters for the sake of user-friendliness and searcheability. See Meta's page on special characters for more details.

Forms

While we're on the tack of users, how do non-UTF-8 web forms deal with characters that our outside of their character set? Rather than discuss what UTF-8 does right, we're going to show what could go wrong if you didn't use UTF-8 and people tried to use characters outside of your character encoding.

The troubles are large, extensive, and extremely difficult to fix (or, at least, difficult enough that if you had the time and resources to invest in doing the fix, you would be probably better off migrating to UTF-8). There are two types of form submission: application/x-www-form-urlencoded which is used for GET and by default for POST, and multipart/form-data which may be used by POST, and is required when you want to upload files.

The following is a summarization of notes from FORM submission and i18n. That document contains lots of useful information, but is written in a rambly manner, so here I try to get right to the point.

application/x-www-form-urlencoded

This is the Content-Type that GET requests must use, and POST requests use by default. It involves the ubiquituous percent encoding format that looks something like: %C3%86. There is no official way of determining the character encoding of such a request, since the percent encoding operates on a byte level, so it is usually assumed that it is the same as the encoding the page containing the form was submitted in. You'll run into very few problems if you only use characters in the character encoding you chose.

However, once you start adding characters outside of your encoding (and this is a lot more common than you may think: take curly "smart" quotes from Microsoft as an example), a whole manner of strange things start to happen. Depending on the browser you're using, they might:

• Replace the unsupported characters with useless question marks,
• Attempt to fix the characters (example: smart quotes to regular quotes),
• Replace the character with a character entity, or
• Send it anyway as a different character encoding mixed in with the original encoding (usually Windows-1252 rather than iso-8859-1 or UTF-8 interspersed in 8-bit)

To properly guard against these behaviors, you'd have to sniff out the browser agent, compile a database of different behaviors, and take appropriate conversion action against the string (disregarding a spate of extremely mysterious, random and devastating bugs Internet Explorer manifests every once in a while). Or you could use UTF-8 and rest easy knowing that none of this could possibly happen since UTF-8 supports every character.

multipart/form-data

Multipart form submission takes a way a lot of the ambiguity that percent-encoding had: the server now can explicitly ask for certain encodings, and the client can explicitly tell the server during the form submission what encoding the fields are in.

There are two ways you go with this functionality: leave it unset and have the browser send in the same encoding as the page, or set it to UTF-8 and then do another conversion server-side. Each method has deficiencies, especially the former.

If you tell the browser to send the form in the same encoding as the page, you still have the trouble of what to do with characters that are outside of the character encoding's range. The behavior, once again, varies: Firefox 2.0 entity-izes them while Internet Explorer 7.0 mangles them beyond intelligibility. For serious I18N purposes, this is not an option.

The other possibility is to set Accept-Encoding to UTF-8, which begs the question: Why aren't you using UTF-8 for everything then? This route is more palatable, but there's a notable caveat: your data will come in as UTF-8, so you will have to explicitly convert it into your favored local character encoding.

I object to this approach on idealogical grounds: you're digging yourself deeper into the hole when you could have been converting to UTF-8 instead. And, of course, you can't use this method for GET requests.

Well supported

Almost every modern browser in the wild today has full UTF-8 and Unicode support: the number of troublesome cases can be counted with the fingers of one hand, and these browsers usually have trouble with other character encodings too. Problems users usually encounter stem from the lack of appropriate fonts to display the characters (once again, this applies to all character encodings and HTML entities) or Internet Explorer's lack of intelligent font picking (which can be worked around).

We will go into more detail about how to deal with edge cases in the browser world in the Migration section, but rest assured that converting to UTF-8, if done correctly, will not result in users hounding you about broken pages.

HTML Purifier

And finally, we get to HTML Purifier.

Migrate to UTF-8

Text editor

Configuring your database

Convert old text

Byte Order Mark (headers already sent!)

Dealing with variable width in functions

Further Reading

Many other developers have already discussed the subject of Unicode, UTF-8 and internationalization, and I would like to defer to them for a more in-depth look into character sets and encodings.

• The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!) by Joel Spolsky, provides a very good high-level look at Unicode and character sets in general.
• UTF-8 on Wikipedia, provides a lot of useful details into the innards of UTF-8, although it may be a little off-putting to people who don't know much about Unicode to begin with.