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

NAME

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

SYNOPSIS

       scan string format ?varName varName ...?
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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.

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.

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.

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, l, and ll. 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 same range produced by the │
int() function of the expr command. The L size modifier is equivalent to the l size │
modifier. Either one indicates the integer range to be stored is limited to the same range │
produced by the wide() function of the expr command. The ll size modifier indicates that │
the integer range to be stored is unlimited.

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.

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.
The conversion makes no sense without reference to a truncation range, so the
size modifier ll is not permitted in combination with conversion character u.

i The input substring must be an integer. The base (i.e. decimal, binary, octal,
or hexadecimal) is determined in the same fashion as described in expr. 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, noting that this avoids problems with octal numbers by  forcing
       interpretation as decimals (if we did not care, we would use the %i conversion instead):
              set string "08:08"   ;# *Not* octal!
              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:                                                                                 │
              % set tcl_platform(wordSize)                                                        │
              4                                                                                   │
              % scan 20000000000000000000 %d                                                      │
              2147483647                                                                          │
              % scan 20000000000000000000 %ld                                                     │
              9223372036854775807                                                                 │
              % scan 20000000000000000000 %lld                                                    │
              20000000000000000000                                                                │

KEYWORDS

       conversion specifier, parse, scan