Tcl Built-In Commands scan(1T)
NAME
scan - Parse string using conversion specifiers in the style
of sscanf
SYNOPSIS
scan string format ?varName varName ...?
INTRODUCTION
This command parses fields from an input string in the same
fashion as 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 field is scanned from string the result is converted back
into a string and 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 SCANING
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 isn't 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 *, which indicates that the converted
value is to be discarded instead of assigned to a variable;
a XPG3 position specifier; a number indicating a maximum
field width; a field size modifier; and a conversion charac-
ter. 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
specifier is [ or c). Then it converts the next input char-
acters 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
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Tcl Built-In Commands scan(1T)
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 following conversion characters are supported:
d The input field must be a decimal integer. It is
read in and the value is stored in the variable as
a decimal string. If the l or L field size modif-
ier is given, the scanned value will have an
internal representation that is at least 64-bits
in size.
o The input field must be an octal integer. It is
read in and the value is stored in the variable as
a decimal string. If the l or L field size modif-
ier is given, the scanned value will have an
internal representation that is at least 64-bits
in size. If the value exceeds MAXINT
(017777777777 on platforms using 32-bit integers
when the l and L modifiers are not given), it will
be truncated to a signed integer. Hence,
037777777777 will appear as -1 on a 32-bit machine
by default.
x The input field must be a hexadecimal integer. It
is read in and the value is stored in the variable
as a decimal string. If the l or L field size
modifier is given, the scanned value will have an
internal representation that is at least 64-bits
in size. If the value exceeds MAXINT (0x7F
on platforms using 32-bit integers when the l and
L modifiers are not given), it will be truncated
to a signed integer. Hence, 0xF will
appear as -1 on a 32-bit machine.
u The input field must be a decimal integer. The
value is stored in the variable as an unsigned
decimal integer string. If the l or L field size
modifier is given, the scanned value will have an
internal representation that is at least 64-bits
in size.
i The input field must be an integer. The base
(i.e. decimal, octal, or hexadecimal) is deter-
mined in the same fashion as described in expr.
The value is stored in the variable as a decimal
string. If the l or L field size modifier is
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Tcl Built-In Commands scan(1T)
given, the scanned value will have an internal
representation that is at least 64-bits in size.
c A single character is read in and its binary value
is stored in the variable as a decimal string.
Initial white space is not skipped in this case,
so the input field may be a white-space character.
This conversion is different from the ANSI stan-
dard in that the input field always consists of a
single character and no field width may be speci-
fied.
s The input field 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 field 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 string.
[chars] The input field consists of any number of charac-
ters 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 field consists of any number of charac-
ters 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 indi-
cating a range.
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.
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Tcl Built-In Commands scan(1T)
The number of characters read from the input for a conver-
sion is the largest number that makes sense for that partic-
ular conversion (e.g. as many decimal digits as possible
for %d, as many octal digits as possible for %o, and so on).
The input field for a given conversion terminates either
when a white-space character is encountered or when the max-
imum field width has been reached, whichever comes first.
If a * is present in the conversion specifier then no vari-
able is assigned and the next scan argument is not consumed.
DIFERENCES FROM ANSI SCANF
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 currently supported.
[2] For %c conversions a single character value is con-
verted to a decimal string, which is then assigned to
the corresponding varName; no field width may be speci-
fied for this conversion.
[3] The h modifier is always ignored and the l and L modif-
iers are ignored when converting real values (i.e. type
double is used for the internal representation).
[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
Parse a simple color specification of the form #RGB using
hexadecimal conversions with field sizes:
set string "#08D03F"
scan $string "#%2x%2x%2x" r g b
Parse a H:M 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 charac-
ters (note the use of the %n conversion so that we get skip-
ping over leading whitespace correct):
set string " a string {with braced words} ] leading space "
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Tcl Built-In Commands scan(1T)
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 com-
plete 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"
SEE ALSO
format(1T), sscanf(3TCL)
KEYWORDS
conversion specifier, parse, scan
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWTcl
Interface Stability Uncommitted
NOTES
Source for Tcl is available on http:/opensolaris.org.
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