scan - Parse string using conversion specifiers in the style of sscanf

Details On Scanning

Scan operates by scanning string and format together. If the next character in format is a blank or tab
then it matches any number of white space characters in string (including zero). Otherwise, if it is not
a % character then it must match the next character of string. When a % is encountered in format, it
indicates the start of a conversion specifier. A conversion specifier contains up to four fields after
the %: a XPG3 position specifier (or a * to indicate the converted value is to be discarded instead of
assigned to any variable); a number indicating a maximum substring width; a size modifier; and a
conversion character. All of these fields are optional except for the conversion character. The fields
that are present must appear in the order given above.

When scan finds a conversion specifier in format, it first skips any white-space characters in string
(unless the conversion character is [ or c). Then it converts the next input characters according to the
conversion specifier and stores the result in the variable given by the next argument to scan.

OPTIONALPOSITIONALSPECIFIER
If the % is followed by a decimal number and a $, as in “%2$d”, then the variable to use is not taken
from the next sequential argument. Instead, it is taken from the argument indicated by the number, where
1 corresponds to the first varName. If there are any positional specifiers in format then all of the
specifiers must be positional. Every varName on the argument list must correspond to exactly one
conversion specifier or an error is generated, or in the inline case, any position can be specified at
most once and the empty positions will be filled in with empty strings.

OPTIONALSIZEMODIFIER
The size modifier field is used only when scanning a substring into one of Tcl's integer values. The
size modifier field dictates the integer range acceptable to be stored in a variable, or, for the inline
case, in a position in the result list. The syntactically valid values for the size modifier are h, l,
z, t, q, j, ll, and L. The h size modifier value is equivalent to the absence of a size modifier in the
the conversion specifier. Either one indicates the integer range to be stored is limited to the 32-bit
range. The L size modifier is equivalent to the ll size modifier. Either one indicates the integer
range to be stored is unlimited. The l (or q or j) size modifier indicates that the integer range to be
stored is limited to the same range produced by the wide() function of the expr command. The z and t
modifiers indicate the integer range to be the same as for either h or l, depending on the value of the
pointerSize element of the tcl_platform array.

MANDATORYCONVERSIONCHARACTER
The following conversion characters are supported:

d The input substring must be a decimal integer. It is read in and the integer value is stored in
the variable, truncated as required by the size modifier value.

o The input substring must be an octal integer. It is read in and the integer value is stored in the
variable, truncated as required by the size modifier value.

x or X The input substring must be a hexadecimal integer. It is read in and the integer value is stored
in the variable, truncated as required by the size modifier value.

b The input substring must be a binary integer. It is read in and the integer value is stored in
the variable, truncated as required by the size modifier value.

u The input substring must be a decimal integer. The integer value is truncated as required by the
size modifier value, and the corresponding unsigned value for that truncated range is computed and
stored in the variable as a decimal string.

i The input substring must be an integer. The base (i.e. decimal, octal, or hexadecimal) is
determined by the C convention (leading 0 for octal; prefix 0x for hexadecimal). The integer
value is stored in the variable, truncated as required by the size modifier value.

c A single character is read in and its Unicode value is stored in the variable as an integer value.
Initial white space is not skipped in this case, so the input substring may be a white-space
character.

s The input substring consists of all the characters up to the next white-space character; the
characters are copied to the variable.

e or f or g or E or G
The input substring must be a floating-point number consisting of an optional sign, a string of
decimal digits possibly containing a decimal point, and an optional exponent consisting of an e or
E followed by an optional sign and a string of decimal digits. It is read in and stored in the
variable as a floating-point value.

[chars]
The input substring consists of one or more characters in chars. The matching string is stored in
the variable. If the first character between the brackets is a ] then it is treated as part of
chars rather than the closing bracket for the set. If chars contains a sequence of the form a-b
then any character between a and b (inclusive) will match. If the first or last character between
the brackets is a -, then it is treated as part of chars rather than indicating a range.

[^chars]
The input substring consists of one or more characters not in chars. The matching string is
stored in the variable. If the character immediately following the ^ is a ] then it is treated as
part of the set rather than the closing bracket for the set. If chars contains a sequence of the
form a-b then any character between a and b (inclusive) will be excluded from the set. If the
first or last character between the brackets is a -, then it is treated as part of chars rather
than indicating a range value.

n No input is consumed from the input string. Instead, the total number of characters scanned from
the input string so far is stored in the variable.

The number of characters read from the input for a conversion is the largest number that makes sense for
that particular conversion (e.g. as many decimal digits as possible for %d, as many octal digits as
possible for %o, and so on). The input substring for a given conversion terminates either when a white-
space character is encountered or when the maximum substring width has been reached, whichever comes
first. If a * is present in the conversion specifier then no variable is assigned and the next scan
argument is not consumed.

Differences From Ansi Sscanf

       The  behavior  of  the scan command is the same as the behavior of the ANSI C sscanf procedure except for
       the following differences:

       [1]    %p conversion specifier is not supported.

       [2]    For %c conversions a single character value is converted  to  a  decimal  string,  which  is  then
              assigned to the corresponding varName; no substring width may be specified for this conversion.

       [3]    The h modifier is always ignored and the l and L modifiers are ignored when converting real values
              (i.e.  type  double  is  used  for  the  internal  representation).  The ll modifier has no sscanf
              counterpart.

       [4]    If the end of the input string is reached before  any  conversions  have  been  performed  and  no
              variables are given, an empty string is returned.

Examples

       Convert a UNICODE character to its numeric value:

              set char "x"
              set value [scan $char %c]

       Parse  a  simple  color  specification  of  the form #RRGGBB using hexadecimal conversions with substring
       sizes:

              set string "#08D03F"
              scan $string "#%2x%2x%2x" r g b

       Parse a HH:MM time string:

              set string "08:08"
              if {[scan $string "%d:%d" hours minutes] != 2} {
                  error "not a valid time string"
              }
              # We have to understand numeric ranges ourselves...
              if {$minutes < 0 || $minutes > 59} {
                  error "invalid number of minutes"
              }

       Break a string up into sequences of non-whitespace characters (note the use of the %n conversion so  that
       we get skipping over leading whitespace correct):

              set string " a string {with braced words} + leading space "
              set words {}
              while {[scan $string %s%n word length] == 2} {
                  lappend words $word
                  set string [string range $string $length end]
              }

       Parse  a  simple coordinate string, checking that it is complete by looking for the terminating character
       explicitly:

              set string "(5.2,-4e-2)"
              # Note that the spaces before the literal parts of
              # the scan pattern are significant, and that ")" is
              # the Unicode character \u0029
              if {
                  [scan $string " (%f ,%f %c" x y last] != 3
                  || $last != 0x0029
              } then {
                  error "invalid coordinate string"
              }
              puts "X=$x, Y=$y"

       An interactive session demonstrating the truncation of integer values determined by size modifiers:

              % scan 20000000000000000000 %d
              2147483647
              % scan 20000000000000000000 %ld
              9223372036854775807
              % scan 20000000000000000000 %lld
              20000000000000000000

Introduction

       This  command  parses substrings from an input string in a fashion similar to the ANSI C sscanf procedure
       and returns a count of the number of conversions performed, or -1 if the  end  of  the  input  string  is
       reached  before  any  conversions  have  been  performed.  String gives the input to be parsed and format
       indicates how to parse it, using % conversion specifiers as in sscanf.  Each varName gives the name of  a
       variable;  when  a substring is scanned from string that matches a conversion specifier, the substring is
       assigned to the corresponding variable.  If no varName variables are specified, then  scan  works  in  an
       inline  manner,  returning  the  data  that would otherwise be stored in the variables as a list.  In the
       inline case, an empty string is returned when  the  end  of  the  input  string  is  reached  before  any
       conversions have been performed.

Keywords

       conversion specifier, parse, scan

Tcl                                                    8.4                                            scan(3tcl)

Name

       scan - Parse string using conversion specifiers in the style of sscanf

Synopsis

scanstringformat ?varNamevarName...?
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