UTF-8 , where UTF is short for Unicode Transformation Format, is a format in which Unicode characters are represented by one to four bytes per character. It is an extension of ascii, uses easily-distinguishable context-free prefixes to disinguish the beginning of each character, and can be probabilistically differentiated from legacy extended ascii encodings.

See Also

Unicode and UTF-8


The interesting characteristics of UTF-8 include:

Valid ASCII is valid UTF-8
Just as Unicode extends the ASCII character map, UTF-8 extends the ASCII character format. In ASCII the top bit of each byte is 0 and any byte in UTF-8 whose top bit is 0 represents that same ASCII character.
In UTF-8 some characters are reprsented by a single byte, while other characters may require up to 4 bytes.
Any byte whose top two bits are not 10 is the first (and maybe only) byte of the next character.
Appending bytes to a valid UTF-8 sequence can not make that sequence invalid.
preserves sort order
Code-point sequences and their utf-8 encodings sort into the same order.
no additional null bytes
Null-terminated utf-8-encoded strings can be handled. However, just as with ASCII, this excludes null from occuring in a string.

Tcl Internals

Internally, Tcl uses modified utf-8 encoding, which is the same as UTF-8 except that the NUL character (\u0000) is encoded as the bytes 0xC0 0x80, which is not a valid UTF-8 sequence. Since there are no nulls in such a string the C-string property that a null byte terminates the string is preserved.

DKF: Here's a little utility procedure I wrote today when I needed to convert the UTF-8 bytes encoding a Unicode character into a sequence hexadecimal digits to use as a literal value in C:

proc toutf8 c {
    set s [encoding convertto utf-8 $c]
    binary scan $s cu* x
    format [string repeat \\x%02x [string length $s]] {*}$x


% toutf8 \u1234
% toutf8 \u0000

ferrieux: May I suggest a slight enhancement of readability, and possibly performance, though not measured nor expecting much:

proc toutf8 c {
    set s [encoding convertto utf-8 $c]
    binary scan $s H* x
    regsub -all -expanded {..} $x {\x&}

The demonstrated output continues to be the same as shown above, as expected.

jima 2010-01-09: Does this work for Unicode characters in the range U+010000 to U+10FFFF ?

U+010000 is xF0 x90 x80 x80

According to [L1 ]

In my box

toutf8 \u10000



And (notice the extra 0 introduced here)

toutf8 \u010000



I have tested some codes in the other ranges defined in [L2 ] and everything seems fine whilst we don't put any extra zeroes at the beginning:

 toutf8 \u20ac

Correctly produces


Lars H, 2010-01-12 PYK 2020-07-22, 2023-01-12: No, Tcl < 9 can only represent characters within the basic multilingual plane, so there's no way that you can even feed an U+10000 character into encoding convertto :-(. Fixing that is non-trivial since some parts of the Tcl C library require a representation of strings where all characters take up the same number of bytes. It is possible to compile Tcl with that TCL_UTF_MAX set to 4, meaning 32 bits per character, but it's rather wasteful, and has been reported not entirely compatible with Tk.

In Tcl < 9 surrogate pairs can be used for characters beyond the BMP, thus treating Tcl's strings as the UTF-16 representations of the strings proper. This doesn't play well with encoding convertto utf-8 though, as that reencodes each surrogate in the pair as a separate character. Perhaps I should get around to doing something about that…

\u by design grabs no more than four hexadecimal digits, leaving extra zeroes alone, and will continue to do so even after Tcl is extended to support full Unicode. This is so that you can put a hex digit immediately after a four-digit \u substitution. This is not possible with \x, which consumes as many hexadecimal digits as it finds. With \U it is possible to represent any Unicode character. regexp already implements that, at least syntactically.

jima, 2010-01-13: Thanks Lars for the explanation.

Perhaps another side of this problem is that even if one is able to generate the right UTF-8 (by coding the Unicode to UTF-8 conversion algorithm oneself), one won't get the proper graphical output unless the instructions to produce it are somewhere in the bellies of Tcl.

So (as I understand it), to display an image of the Unicode character U+10000, properly encoded internally as UTF-8 as \xF0\x90\x80\x80, we would need extra information besides the algorithm depicted in the wikipedia page mentioned earlier.

Lars H, 2010-01-19, 2010-01-26: The graphical output always depends on what is supposed to be supplying it. Tk is probably difficult, but if you're rather generating text that some other program (e.g. a web browser) is supposed to render, then 4-byte UTF-8 sequences may be fine.

For what it's worth, I went ahead with the "surrogate pairs inside Tcl — 4-byte sequences outside" idea; the result so far can be found in the Half Bakery at https://wiki.tcl-lang.org/_repo/UTF/ . This is an C-coded extension (well, the files needed for one which are not the same as in the sampleextension), whose package name is UTF, and which defines a new encoding, "UTF-8" (upper case, whereas the built-in one is utf-8). Upon encoding convertfrom, this converts 4-byte sequences (code points U+10000 through U+10FFFF) to surrogate pairs, and upon encoding convertto it converts surrogate pairs to 4-byte sequences. There are even tests, which it passes! A previous version (from 2010-01-19) had a bug that caused it to get stuck in an infinite loop when used as a channel encoding, but the current seems to work fine.

A logical next step would be to also implement UTF-16BE and UTF-16LE as encodings (the Unicode built-in encoding is almost one of these, but it depends on the platform which one it is).

AMG: See #Feature Request #392, New encodings: utf-16, utf-16be, utf-16le for the UTF-16 encoding feature request.