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scan(n)			     Tcl Built-In Commands		       scan(n)

______________________________________________________________________________

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

SYNOPSIS
       scan string format ?varName varName ...?
_________________________________________________________________

INTRODUCTION
       This  command parses substrings from an input string in a fashion simi‐
       lar 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 spec‐
       ifier, 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  conver‐
       sion  specifier.	  A  conversion	 specifier  contains up to four fields
       after the %: a XPG3 position specifier (or a *  to  indicate  the  con‐
       verted 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	speci‐
       fiers  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 lim‐ │
       ited to the same range produced by the int() function of the expr  com‐ │
       mand.  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	 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  trunca‐
		 tion  range, so the size modifier ll is not permitted in com‐
		 bination with conversion character u.

       i	 The input substring must be an integer.  The base (i.e. deci‐
		 mal, 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
		 The  input substring must be a floating-point number consist‐
		 ing of an optional sign, a string of decimal digits  possibly
		 containing a decimal point, and an optional exponent consist‐
		 ing 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 pos‐
       sible 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 var‐
	      Name; 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  hexadeci‐
       mal 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	white‐
       space 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 look‐
       ing 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					       │

SEE ALSO
       format(n), sscanf(3)

KEYWORDS
       conversion specifier, parse, scan

Tcl				      8.4			       scan(n)
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