[Richard Suchenwirth] 2004-05-12 - "Quoted-printable" is a way of encoding 8-bit characters by using only 7-bit ASCII characters, by replacing them with `=XX`, `XX` being two hex digits. 
This is used for instance in MIME mail attachments ( http://www.faqs.org/rfcs/rfc2045.html has many details). 
Decoding this type of data goes in a one-liner with [subst]:

======
 proc quoted-printable s {subst -nocom -novar [string map {\\ \\\\ =\n "" = \\u00} $s]}
======

The other way it seems to need explicit iteration:

======
 proc to-quoted-printable s {
    set res ""
    foreach char [split $s ""] {
        scan $char %c i
        if {$i>255} {error "character $i does not fit in 8 bits"}
        append res [expr {$i<128? $char: [format =%02X $i]}]
    }
    set res
 }
# Test and demo:
 % to-quoted-printable "Schöne Grüße"
 Sch=F6ne Gr=FC=DFe
======

Note that `\u00XX` was used in the decoder in place of `\xXX`, which is "greedy" and 
takes as many hex characters as it can get, working only on the last two of them.

[Lars H]: I added a doubling of the backslashes in [[quoted-printable]], 
to prevent unwanted substitutions. Also, I think [[to-quoted-printable]] 
should quote = as well as the non-ASCII characters.

For moderately long texts, it should be better to do it as follows
======
 proc to-quoted-printable s {
    set QP [list = =3D]
    for {set i 128} {$i<256} {incr i} {
       lappend QP [format %c $i] [format =%02X $i]
    }
    return [string map $QP $s]
 }
======

----

[daapp]: I found that package [mime] v1.3.6 from [tcllib]1.6 contain two undocumented, 
but very useful procedures: '''mime::word_encode''' and '''mime::word_decode''' 
which encode and decode string as described in [MIME -- Multipurpose Internet Mail Extensions] part three. Example: 

    foreach {charset encoding decodedString} [mime::decode encodedString] break
    set originalString [encoding convertfrom $charset $decodedString]

Where:
   * charset - name of charset of decodedString.
   * encoding - base64 or quoted-printable
   * decodedString - you know

After decoding you need to convert string to specified charset.

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