See
http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/scan.htm 
for the formal man page for the [Tcl] command scan.

 scan $possiblyZeroedDecimal %d cleanInteger
Context: Leading zeroes induce octal parsing in the [expr] command and in 
Context: Leading zeroes induce octal parsing in the expr command and in
Use of scan to remove leading zeros can avoid this often-reported problem.

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display and numeric format for ASCII characters.  
display and numeric format for ASCII characters.  Scan provides the
usual answer:
    foreach character {a A b B c} {
        scan $character %c numeric
        puts "ASCII character '$numeric' displays as '$character'."
    }

    ASCII character '97' displays as 'a'.
    ASCII character '65' displays as 'A'.
    ASCII character '98' displays as 'b'.
    ASCII character '66' displays as 'B'.
    ASCII character '99' displays as 'c'.        
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In more recent Tcl's (8.3 or so up) you also can call [scan] inline:
 set numeric [scan $character %c]
And the %c format is not limited to ASCII, but can handle any Unicode. ([RS])

[LV] Can someone explain what this means - calling scan inline?  Where is
scan getting its input in this case?  From [stdin]?
** Unmatched Conversion Specifiers **
[Lars H]: It refers to the result of scan, not its input. Rather than setting some variables, it will return a list of the parsed values. (RS's example is rather ugly, since it really sets $numeric to a list with one element, but as quick character code look-up at a prompt, a '''scan''' ''char'' '''%c''' is hard to beat.)
[Alastair Davies]/[PYK]: When `scan` returns a list of matching values, it includes an empty string for each format specifier that wasn't matched. If there is only one format specifier, a list with one blank element is returned, and this is not the same thing as and empty string. For example, '''scan foo %d''' returns `{}`, which not an empty string, but a literal open-brace character followed by a literal close-brace character.   The documentation states:
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Also see "[Dump a file in hex and ASCII]", as well as
"u2x" in the "[Bag of algorithms]".

[Binary] has a scan which complements use of this scan.
** Effect on the Internal Representation of a Value **

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[Tcl syntax help] - 
[Arts and crafts of Tcl-Tk programming] - 
[Category Command] -
[Category String Processing]