scan - Parse string using conversion specifiers in the style of sscanf
scan string format ?
varName varName ...?
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.
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.
- 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, 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.
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.
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
format(3tcl),
sscanf(3)
conversion specifier, parse, scan