Provided by: tcl8.6-doc_8.6.1-4ubuntu1_all 
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.
OPTIONAL POSITIONAL SPECIFIER
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.
OPTIONAL SIZE MODIFIER
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.
MANDATORY CONVERSION CHARACTER
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.
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. 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
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
SEE ALSO
format(3tcl), sscanf(3)
KEYWORDS
conversion specifier, parse, scan