According to http://en.wikipedia.org/wiki/UTF-8.
11011111 11111111
is an illegal utf-8 byte sequence.
How will it been parsed?
and
How does vim, iconv, java... deal with illegal utf-8 byte sequence?
UTF-8 is a transformation format to derive a subset of the Universal Character Set (UCS). This transformation and decoding of such characters is governed by rules defined as Internet Standards.
In section 3 of RFC3629, it clearly says:
Implementations of the decoding algorithm above MUST protect against
decoding invalid sequences.
It has no specific instructions on how to handle such invalid sequences other that "protect against", it depends on the nature of implementation and implementers what specific actions will be taken.
Related
A long time ago, there was a two-byte Unicode encoding UCS-2, but then it was determined that two bytes are sometimes not enough. In order to cram more codepoints into 16 bit, surrogate pairs were introduced in UTF-16. Since Windows started out with UCS-2, it doesn't enforce rules around surrogate pairs in some places, most notably file systems.
Programs that want to use UTF-8 internally have a problem now dealing with these invalid UTF-16 sequences. For this, WTF-8 was developed. It is mostly relaxed UTF-8, but it is able to round-trip invalid surrogate pairs.
Now it seems like it should be possible to relax UTF-8 a bit further, and allow it to represent arbitrary binary data, round-tripping safe. The strings I am thinking about are originally 99.9% either valid UTF-8, or almost valid UTF-16 of the kind WTF-8 can stomach. But occasionally there will be invalid byte sequences thrown in.
WTF-8 defines generalized UTF-8 as:
an encoding of sequences of code points (not restricted to Unicode scalar values) using 8-bit bytes, based on the same underlying algorithm as UTF-8. It is a strict superset of UTF-8 (like UTF-8 is a strict superset of ASCII).
Would generalized UTF-8 allow me to store arbitrary 32 bit sequences, and thus arbitrary data? Or is there another way, such as a unicode escape character? Things I don't want to do are base64 encoding or percent-encoding, since I want to leave valid unicode strings unchanged.
Standard disclaimer: I encountered this problem a couple times before, but right now it is an academic question, and I'm just interested in a straight answer how to do this. There is no XY problem :-)
I'm writing a UTF-8 decoder, and I don't know how to handle invalid codepoints correctly:
surrogates
codepoints larger than 0x10ffff
Suppose, that I'd like to replace invalid codepoints with 0xfffd. Now, how should I replace them? Immediately after I know that the codepoint cannot be valid, or should I decode/consume all bytes that the first byte mandates?
For example, suppose, that the UTF-8 stream contains: 0xf4 0x90 0x80 0x80
These bytes decode to 0x110000, an invalid codepoint.
But, at the second byte, when 0xf4 0x90 is processed, I know, that it cannot be a valid codepoint, no matter what the last two bytes are.
So, should this stream generate one error (and one replacement), or should it generate 3 errors (because 0xf4 0x90 is invalid, and then 0x80 and the other 0x80 is invalid as well)?
Is there a standard which mandates this? If not, what could be the best practice?
I've found an answer in the unicode standard, chapter 03, pages 126-129:
unicode standard mandates that a well-formed subsequence must not be consumed as a part of ill-formed sequence (my example doesn't contain such a case, though)
there is a recommendation to follow W3C: one error should be generated for a maximal subpart of an ill-formed subsequence (see the definition in the linked document)
the second byte of 0xf4 0x90 0x80 0x80 is invalid, so I should generate 4 errors if the recommendation is followed (because the 2nd byte is invalid, the maximal subpart at the beginning is just 0xf4)
If my example have been 0xf4 0x8f 0x41, then I should generate 1 error only, as 0xf4 0x8f is a maximal subpart, and 0x41 is a well-formed subsequence.
The Unicode Consortium seems to be concerned only with accuracy (not dropping good bytes) and security (not putting two pieces of good text together because a security scanner might have given the text a pass when considering the bad bytes but perhaps would have blocked the cleaned up text). It allows others to define any specific practices. (It seems that had proposed best practices but will back away since W3C has formalized them.)
W3C is concerned with security, of course, but also with the consistency you ask for. It says to error (e.g. insert the replacement character) for every ill-formed subsequence, per its very detailed reference UTF-8 Decoder algorithm.
I have a strange validation program that validates wheather a utf-8 string is a valid host name(Zend Framework Hostname valdiator in PHP). It allows IDNs(internationalized domain names). It will compare each subdomain with sets of characters defined by their HEX bytes representation. Two such sets are D800-DB7F and DC00-DFFF. Php regexp comparing function called preg_match fails during these comparsions and it says that DC00-DFFF characters are not allowed in this function. From wikipedia I learned these bytes are called surrogate characters in UTF-8. What are thay and which characters they actually correspond to? I read in several places I still don't understand what they are.
What are surrogate characters in UTF-8?
This is almost like a trick question.
Approximate answer #1: 4 bytes (if paired and encoded in UTF-8).
Approximate answer #2: Invalid (if not paired).
Approximate answer #3: It's not UTF-8; It's Modified UTF-8.
Synopsis: The term doesn't apply to UTF-8.
Unicode codepoints have a range that needs 21 bits of data.
UTF-16 code units are 16 bits. UTF-16 encodes some ranges of Unicode codepoints as one code unit and others as pairs of two code units, the first from a "high" range, the second from a "low" range. Unicode reserves the codepoints that match the ranges of the high and low pairs as invalid. They are sometimes called surrogates but they are not characters. They don't mean anything by themselves.
UTF-8 code units are 8 bits. UTF-8 encodes several distinct ranges of codepoints in one to four code units, respectively.
#1 It happens that the codepoints that UTF-16 encodes with two 16-bit code units, UTF-8 encodes with 4 8-bit code units, and vice versa.
#2 You can apply the UTF-8 encoding algorithm to the invalid codepoints, which is invalid. They can't be decoded to a valid codepoint. A compliant reader would throw an exception or throw out the bytes and insert a replacement character (�).
#3 Java provides a way of implementing functions in external code with a system called JNI. The Java String API provides access to String and char as UTF-16 code units. In certain places in JNI, presumably as a convenience, string values are modified UTF-8. Modified UTF-8 is the UTF-8 encoding algorithm applied to UTF-16 code units instead of Unicode codepoints.
Regardless, the fundamental rule of character encodings is to read with the encoding that was used to write. If any sequence of bytes is to be considered text, you must know the encoding; Otherwise, you have data loss.
I was solving below problem while reading its solution in first line I read this
can anyone help me in explaining assume char set is ASCII **I Don't want any other solution for this problem I just want to understand the statement **
Implement an algorithm to determine if a string has all unique characters. What if you can not use additional data structures
Thanks in advance for the help.
There is no text but encoded text.
Text is a sequence of "characters", members of a character set. A character set is a one-to-one mapping between a notional character and a non-negative integer, called a codepoint.
An encoding is a mapping between a codepoint and a sequence of bytes.
Examples:
ASCII, 128 codepoints, one encoding
OEM437, 256 codepoints, one encoding
Windows-1252, 251 codepoints, one encoding
ISO-8859-1, 256 codepoints, one encoding
Unicode, 1,114,112 codepoints, many encodings: UTF-8, UTF-16, UTF-32,…
When you receive a byte stream or read a file that represents text, you have to know the character set and encoding. Conversely, when you send a byte stream or write a file that represents text, you have let the receiver know the character set and encoding. Otherwise, you have a failed communication.
Note: Program source code is almost always text files. So, this communication requirement also applies between you, your editor/IDE and your compiler.
Note: Program console input and output are text streams. So, this communication requirement also applies between the program, its libraries and your console (shell). Go locale or chcp to find out what the encoding is.
Many character sets are a superset of ASCII and some encodings map the same characters with the same byte sequences. This causes a lot of confusion, limits learning, promotes usage of poor terminology and the partial interoperablity leads to buggy code. A deliberate approach to specifications and coding eliminates that.
Examples:
Some people say "ASCII" when they mean the common subset of characters between ASCII and the character set they are actually using. In Unicode and elsewhere this is called C0 Controls and Basic Latin.
Some people say "ASCII Code" when they just mean codepoint or the codepoint's encoded bytes (or code units).
The context of your question is unclear but the statement is trying to say that the distinct characters in your data are in the ASCII character set and therefore their number is less than or equal to 128. Due to the similarity between character sets, you can assume that the codepoint range you need to be concerned about is 0 to 127. (Put comments, asserts or exceptions as applicable in your code to make that clear to readers and provide some runtime checking.)
What this means in your programming language depends on the programming language and its libraries. Many modern programming languages use UTF-16 to represent strings and UTF-8 for streams and files. Programs are often built with standard libraries that account for the console's encoding (actual or assumed) when reading or writing from the console.
So, if your data comes from a file, you must read it using the correct encoding. If your data comes from a console, your program's standard libraries will possibly change encodings from the console's encoding to the encoding of the language's or standard library's native character and string datatypes. If your data comes from a source code file, you have to save it in one specific encoding and tell the compiler what that is. (Usually, you would use the default source code encoding assumed by the compiler because that generally doesn't change from system to system or person to person.)
The "additional" data structures bit probably refers to what a language's standard libraries provide, such as list, map or dictionary. Use what you've been taught so far, like maybe just an array. Of course, you can just ask.
Basically, assume that character codes will be within the range 0-127. You won't need to deal with crazy accented characters.
More than likely though, they won't use many, if any codes below 32; since those are mostly non-printables.
Characters such as 'a' 'b' '1' or '#' are encoded into a binary number when stored and used by a computer.
e.g.
'a' = 1100001
'b' = 1100010
There are a number of different standards that you could use for this encoding. ASCII is one of those standards. The other most common standard is called UTF-8.
Not all characters can be encoded by all standards. ASCII has a much more limited set of characters than UTF-8. As such an encoding also defines the set of characters "char set" that are supported by that encoding.
ASCII encodes each character into a single byte. It supports the letters A-Z, and lowercase a-z, the digits 0-9, a small number of familiar symbols, and a number of control characters that were used in early communication protocols.
The full set of characters supported by ASCII can be seen here: https://en.wikipedia.org/wiki/ASCII
Does the term multibyte refer to a charset whose characters can - but don't have to be - wider than 1 byte, (e.g. UTF-8) or does it refer to character sets which are in any case wider than 1 byte (e.g. UTF-16) ? In other words: What is meant if anybody talks about multibyte character sets?
The term is ambiguous, but in my internationalization work, we typically avoided the term "multibyte character sets" to refer to Unicode-based encodings. Generally, we used the term only for legacy encoding schemes that had one or more bytes to define each character (excluding encodings that require only one byte per character).
Shift-jis, jis, euc-jp, euc-kr, along with Chinese encodings are typically included.
Most of the legacy encodings, with some exceptions, require a sort of state machine model (or, more simply, a page swapping model) to process, and moving backwards in a text stream is complicated and error-prone. UTF-8 and UTF-16 do not suffer from this problem, as UTF-8 can be tested with a bitmask and UTF-16 can be tested against a range of surrogate pairs, so moving backward and forward in a non-pathological document can be done safely without major complexity.
A few legacy encodings, for languages like Thai and Vietnamese, have some of the complexity of multibyte character sets but are really just built on combining characters, and aren't generally lumped in with the broad term "multibyte."
What is meant if anybody talks about multibyte character sets?
That, as usual, depends on who is doing the talking!
Logically, it should include UTF-8, Shift-JIS, GB etc.: the variable-length encodings. UTF-16 would often not be considered in this group (even though it kind of is, what with the surrogates; and certainly it's multiple bytes when encoded into bytes via UTF-16LE/UTF-16BE).
But in Microsoftland the term would more typically be used to mean a variable-length default system codepage (for legacy non-Unicode applications, of which there are sadly still plenty). In this usage, UTF-8 and UTF-16LE/UTF-16BE cannot be included because the system codepage on Windows cannot be set to either of these encodings.
Indeed, in some cases “mbcs” is no more than a synonym for the system codepage, otherwise known (even more misleadingly) as “ANSI”. In this case a “multibyte” character set could actually be something as trivial as cp1252 Western European, which only uses one byte per character!
My advice: use “variable-length” when you mean that, and avoid the ambiguous term “multibyte”; when someone else uses it you'll need to ask for clarification, but typically someone with a Windows background will be talking about a legacy East Asian codepage like cp932 (Shift-JIS) and not a UTF.
All character sets where you dont have a 1 byte = 1 character mapping. All Unicode variants, but also asian character sets are multibyte.
For more information, I suggest reading this Wikipedia article.
A multibyte character will mean a character whose encoding requires more than 1 byte. This does not imply however that all characters using that particular encoding will have the same width (in terms of bytes). E.g: UTF-8 and UTF-16 encoded character may use multiple bytes sometimes whereas all UTF-32 encoded characters always use 32-bits.
References:
IBM: Multibyte Characters
Unicode and MultiByte Character Set (archived), Unicode and Multibyte Character Set (MBCS) Support | Microsoft Docs
Unicode Consortium Website
A multibyte character set may consist of both one-byte and two-byte
characters. Thus a multibyte-character string may contain a mixture of
single-byte and double-byte characters.
Ref: Single-Byte and Multibyte Character Sets
UTF-8 is multi-byte, which means that each English character (ASCII) is stored in 1 byte while non-english character like Chinese, Thai, is stored in 3 bytes. When you mix Chinese/Thai with English, like "ทt", the first Thai character "ท" uses 3 bytes while the second English character "t" uses only 1 byte. People who designed multi-byte encoding realized that English character shouldn't be stored in 3 bytes while it can fit in 1 byte due to the waste of storage space.
UTF-16 stores each character either English or non-English in a fixed 2 byte length so it is not multi-byte but called a wide character. It is very suitable for Chinese/Thai languages where each character fits entirely in 2 bytes but printing to utf-8 console output need a conversion from wide character to multi-byte format by using function wcstombs().
UTF-32 stores each character in a fixed 4 byte length but nobody use it to store character due to a waste of storage space.
Typically the former, i.e. UTF-8-like. For more info, see Variable-width encoding.
The former - although the term "variable-length encoding" would be more appropriate.
I generally use it to refer to any character that can have more than one byte per character.