Standard ISO 10646 defines a universal character set, intended to encompass in the long run all languages written on this planet. It is based on wide characters, and offer possibilities for two billion characters (2^31).
This charset was to become available in recode
under the name
UCS
, with many external surfaces for it. But in the current
version, only surfaces of UCS
are offered, each presented as a
genuine charset rather than a surface. Such surfaces are only meaningful
for the UCS
charset, so it is not that useful to draw a line
between the surfaces and the only charset to which they may apply.
UCS
stands for Universal Character Set. UCS-2
and
UCS-4
are fixed length encodings, using two or four bytes per
character respectively. UTF
stands for UCS
Transformation
Format, and are variable length encodings dedicated to UCS
.
UTF-1
was based on ISO 2022, it did not succeed9. UTF-2
replaced it, it has been called UTF-FSS
(File System Safe) in
Unicode or Plan9 context, but is better known today as UTF-8
.
To complete the picture, there is UTF-16
based on 16 bits bytes,
and UTF-7
which is meant for transmissions limited to 7-bit bytes.
Most often, one might see UTF-8
used for external storage, and
UCS-2
used for internal storage.
When recode
is producing any representation of UCS
,
it uses the replacement character U+FFFD
for any valid
character which is not representable in the goal charset10.
This happens, for example, when UCS-2
is not capable to echo a
wide UCS-4
character, or for a similar reason, an UTF-8
sequence using more than three bytes. The replacement character is
meant to represent an existing character. So, it is never produced to
represent an invalid sequence or ill-formed character in the input text.
In such cases, recode
just gets rid of the noise, while taking note
of the error in its usual ways.
Even if UTF-8
is an encoding, really, it is the encoding of a single
character set, and nothing else. It is useful to distinguish between an
encoding (a surface within recode
) and a charset, but only
when the surface may be applied to several charsets. Specifying a charset
is a bit simpler than specifying a surface in a recode
request.
There would not be a practical advantage at imposing a more complex syntax
to recode
users, when it is simple to assimilate UTF-8
to
a charset. Similar considerations apply for UCS-2
, UCS-4
,
UTF-16
and UTF-7
. These are all considered to be charsets.
• UCS-2 | Universal Character Set, 2 bytes | |
• UCS-4 | Universal Character Set, 4 bytes | |
• UTF-7 | Universal Transformation Format, 7 bits | |
• UTF-8 | Universal Transformation Format, 8 bits | |
• UTF-16 | Universal Transformation Format, 16 bits | |
• count-characters | Frequency count of characters | |
• dump-with-names | Fully interpreted UCS dump |
One surface of UCS
is usable for the subset defined by its first
sixty thousand characters (in fact, 31 * 2^11 codes), and uses
exactly two bytes per character. It is a mere dump of the internal
memory representation which is natural for this subset and as such,
conveys with it endianness problems.
A non-empty UCS-2
file normally begins with a so called byte
order mark, having value 0xFEFF
. The value 0xFFFE
is not an
UCS
character, so if this value is seen at the beginning of a file,
recode
reacts by swapping all pairs of bytes. The library also
properly reacts to other occurrences of 0xFEFF
or 0xFFFE
elsewhere than at the beginning, because concatenation of UCS-2
files should stay a simple matter, but it might trigger a diagnostic
about non canonical input.
By default, when producing an UCS-2
file, recode
always
outputs the high order byte before the low order byte. But this could be
easily overridden through the 21-Permutation
surface
(see Permutations). For example, the command:
recode u8..u2/21 < input > output
asks for an UTF-8
to UCS-2
conversion, with swapped byte
output.
Use UCS-2
as a genuine charset. This charset is available in
recode
under the name ISO-10646-UCS-2
. Accepted aliases
are UCS-2
, BMP
, rune
and u2
.
The recode
library is able to combine UCS-2
some sequences
of codes into single code characters, to represent a few diacriticized
characters, ligatures or diphtongs which have been included to ease
mapping with other existing charsets. It is also able to explode
such single code characters into the corresponding sequence of codes.
The request syntax for triggering such operations is rudimentary and
temporary. The combined-UCS-2
pseudo character set is a special
form of UCS-2
in which known combinings have been replaced by the
simpler code. Using combined-UCS-2
instead of UCS-2
in an
after position of a request forces a combining step, while using
combined-UCS-2
instead of UCS-2
in a before position
of a request forces an exploding step. For the time being, one has to
resort to advanced request syntax to achieve other effects. For example:
recode u8..co,u2..u8 < input > output
copies an UTF-8
input over output, still to be in
UTF-8
, yet merging combining characters into single codes whenever
possible.
Another surface of UCS
uses exactly four bytes per character, and is
a mere dump of the internal memory representation which is natural
for the whole charset and as such, conveys with it endianness problems.
Use it as a genuine charset. This charset is available in recode
under the name ISO-10646-UCS-4
. Accepted aliases are UCS
,
UCS-4
, ISO_10646
, 10646
and u4
.
UTF-7
comes from IETF rather than ISO, and is described by RFC 2152, in the MIME series. The UTF-7
encoding is meant to fit
UCS-2
over channels limited to seven bits per byte. It proceeds
from a mix between the spirit of Quoted-Printable
and methods of
Base64
, adapted to Unicode contexts.
This charset is available in recode
under the name
UNICODE-1-1-UTF-7
. Accepted aliases are UTF-7
, TF-7
and u7
.
Even if UTF-8
does not originally come from IETF, there is now
RFC 2279 to describe it. In letters sent on 1995-01-21 and 1995-04-20,
Markus Kuhn writes:
UTF-8
is anASCII
compatible multi-byte encoding of the ISO 10646 universal character set (UCS
).UCS
is a 31-bit superset of all other character set standards. The first 256 characters ofUCS
are identical to those of ISO 8859-1 (Latin-1). TheUCS-2
encoding of UCS is a sequence of bigendian 16-bit words, theUCS-4
encoding is a sequence of bigendian 32-bit words. TheUCS-2
subset of ISO 10646 is also known as “Unicode”. As bothUCS-2
andUCS-4
require heavy modifications to traditionalASCII
oriented system designs (e.g. Unix), theUTF-8
encoding has been designed for these applications.In
UTF-8
, onlyASCII
characters are encoded using bytes below 128. All other non-ASCII characters are encoded as multi-byte sequences consisting only of bytes in the range 128-253. This avoids critical bytes like NUL and / inUTF-8
strings, which makes theUTF-8
encoding suitable for being handled by the standard C string library and being used in Unix file names. Other properties include the preserved lexical sorting order and thatUTF-8
allows easy self-synchronisation of software receivingUTF-8
strings.
UTF-8
is the most common external surface of UCS
, each
character uses from one to six bytes, and is able to encode all 2^31
characters of the UCS
. It is implemented as a charset, with the
following properties:
ASCII
is completely invariant under UTF-8
,
and those are the only one-byte characters. UCS
values and
ASCII
values coincide. No multi-byte characters ever contain bytes
less than 128. NUL
is NUL
. A multi-byte character
always starts with a byte of 192 or more, and is always followed by a
number of bytes between 128 to 191. That means that you may read at
random on disk or memory, and easily discover the start of the current,
next or previous character. You can count, skip or extract characters
with this only knowledge.
UTF-8
, or to safely state that it is not.
These properties also have a few nice consequences:
UCS
representation. Here, N is a number between
1 and 6. So, UTF-8
is most economical when mapping ASCII (1 byte),
followed by UCS-2
(1 to 3 bytes) and UCS-4
(1 to 6 bytes).
UCS
strings is preserved.
In some case, when little processing is done on a lot of strings, one may
choose for efficiency reasons to handle UTF-8
strings directly even
if variable length, as it is easy to get start of characters. Character
insertion or replacement might require moving the remainder of the string
in either direction. In most cases, it is faster and easier to convert
from UTF-8
to UCS-2
or UCS-4
prior to processing.
This charset is available in recode
under the name UTF-8
.
Accepted aliases are UTF-2
, UTF-FSS
, FSS_UTF
,
TF-8
and u8
.
Next: count-characters, Previous: UTF-8, Up: Universal [Contents][Index]
Another external surface of UCS
is also variable length, each
character using either two or four bytes. It is usable for the subset
defined by the first million characters (17 * 2^16) of UCS
.
Martin J. Dürst writes (to comp.std.internat, on 1995-03-28):
UTF-16
is another method that reserves two times 1024 codepoints in Unicode and uses them to index around one million additional characters.UTF-16
is a little bit like former multibyte codes, but quite not so, as both the first and the second 16-bit code clearly show what they are. The idea is that one million codepoints should be enough for all the rare Chinese ideograms and historical scripts that do not fit into the Base Multilingual Plane of ISO 10646 (with just about 63,000 positions available, now that 2,000 are gone).
This charset is available in recode
under the name UTF-16
.
Accepted aliases are Unicode
, TF-16
and u6
.
Next: dump-with-names, Previous: UTF-16, Up: Universal [Contents][Index]
A device may be used to obtain a list of characters in a file, and how many
times each character appears. Each count is followed by the UCS-2
value of the character and, when known, the RFC 1345 mnemonic for that
character.
This charset is available in recode
under the name
count-characters
.
This count
feature has been implemented as a charset. This may
change in some later version, as it would sometimes be convenient to count
original bytes, instead of their UCS-2
equivalent.
Previous: count-characters, Up: Universal [Contents][Index]
Another device may be used to get fully interpreted dumps of an UCS-2
stream of characters, with one UCS-2
character displayed on a full
output line. Each line receives the RFC 1345 mnemonic for the character
if it exists, the UCS-2
value of the character, and a descriptive
comment for that character. As each input character produces its own
output line, beware that the output file from this conversion may be much,
much bigger than the input file.
This charset is available in recode
under the name
dump-with-names
.
This dump-with-names
feature has been implemented as a charset rather
than a surface. This is surely debatable. The current implementation
allows for dumping charsets other than UCS-2
. For example, the
command ‘recode l2..full < input’ implies a necessary
conversion from Latin-2
to UCS-2
, as dump-with-names
is only connected out from UCS-2
. In such cases, recode
does not display the original Latin-2
codes in the dump, only the
corresponding UCS-2
values. To give a simpler example, the command
echo 'Hello, world!' | recode us..dump
produces the following output:
UCS2 Mne Description 0048 H latin capital letter h 0065 e latin small letter e 006C l latin small letter l 006C l latin small letter l 006F o latin small letter o 002C , comma 0020 SP space 0077 w latin small letter w 006F o latin small letter o 0072 r latin small letter r 006C l latin small letter l 0064 d latin small letter d 0021 ! exclamation mark 000A LF line feed (lf)
The descriptive comment is given in English and ASCII
,
yet if the English description is not available but a French one is, then
the French description is given instead, using Latin-1
. However,
if the LANGUAGE
or LANG
environment variable begins with
the letters ‘fr’, then listing preference goes to French when both
descriptions are available.
Here is another example. To get the long description of the code 237 in Latin-5 table, one may use the following command.
echo -n 237 | recode l5/d..dump
If your echo
does not grok ‘-n’, use ‘echo 237\c’ instead.
Here is how to see what Unicode U+03C6
means, while getting rid of
the title lines.
echo -n 0x03C6 | recode u2/x2..dump | tail +3
It is not
probable that recode
will ever support UTF-1
.
This is when the goal charset allows for 16-bits. For shorter charsets, the ‘--strict’ (‘-s’) option decides what happens: either the character is dropped, or a reversible mapping is produced on the fly.
Previous: count-characters, Up: Universal [Contents][Index]