Charset detection

Character encoding detection, charset detection, or code page detection is the process of heuristically guessing the character encoding of a series of bytes that represent text. The technique is recognised to be unreliable and is only used when specific metadata, such as a HTTP header is either not available, or is assumed to be untrustworthy.

This algorithm usually involves statistical analysis of byte patterns; such statistical analysis can also be used to perform language detection. This process is not foolproof because it depends on statistical data.

In general, incorrect charset detection leads to mojibake.

One of the few cases where charset detection works reliably is detecting UTF-8. This is due to the large percentage of invalid byte sequences in UTF-8,In a ''random'' byte string, a byte with the high bit set has only a 1/15 chance of starting a valid UTF-8 code point. Odds are even lower in actual text, which is not random but tends to contain isolated bytes with the high bit set which are always invalid in UTF-8. so that text in any other encoding that uses bytes with the high bit set is ''extremely'' unlikely to pass a UTF-8 validity test. However, badly written charset detection routines do not run the reliable UTF-8 test first, and may decide that UTF-8 is some other encoding. For example, websites in UTF-8 containing the name of the German city München may display , due to the code deciding that the encoding was ISO-8859-1 or Windows-1252 before (or without) even testing to see if it was UTF-8.

UTF-16 is fairly reliable to detect due to the high number of newlines (U+000A) and spaces (U+0020) that should be found when dividing the data into 16-bit words, and large numbers of NUL bytes all at even or odd locations. Common characters ''must'' be checked for, relying on a test to see that the text is valid UTF-16 fails: the Windows operating system would mis-detect the phrase " Bush hid the facts" (without a newline) in ASCII as Chinese UTF-16LE, since all the byte pairs matched assigned Unicode characters in UTF-16LE.

Charset detection is particularly unreliable in Europe, in an environment of mixed ISO-8859 encodings. These are closely related eight-bit encodings that share an overlap in their lower half with ASCII and all arrangements of bytes are valid. There is no technical way to tell these encodings apart and recognizing them relies on identifying language features, such as letter frequencies or spellings.

Due to the unreliability of heuristic detection, it is better to properly label datasets with the correct encoding. See Character encodings in HTML#Specifying the document's character encoding. Even though UTF-8 and UTF-16 are easy to detect, some systems require UTF encodings to explicitly label the document with a prefixed byte order mark (BOM).

See also

* International Components for Unicode – a library that can perform charset detection
* Language identification
* Content sniffing
* Browser sniffing – a similar heuristic technique for determining the capabilities of a web browser, before serving content to it

References

External links

IMultiLanguage2::DetectInputCodepage



Reference for ''cpdetector'' charset detection



Java port of Mozilla Charset Detectors

Delphi/Pascal port of Mozilla Charset Detectors

''uchardet'', C++ fork of Mozilla Charset Detectors; includes Bash command-line tool

C# port of Mozilla Charset Detectors

HEBCI, a technique for detecting the character set used in form submissions

Frequency distributions of English trigraphs

{{character encoding, state=collapsed

Character encoding