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# scanf for Ruby
# $Release Version: 1.1.2 $
# $Revision: 19094 $
# $Id: scanf.rb 19094 2008-09-03 12:54:13Z dblack $
# $Author: dblack $
# A product of the Austin Ruby Codefest (Austin, Texas, August 2002)
=scanf for Ruby
scanf for Ruby is an implementation of the C function scanf(3),
modified as necessary for Ruby compatibility.
The methods provided are String#scanf, IO#scanf, and
Kernel#scanf. Kernel#scanf is a wrapper around STDIN.scanf. IO#scanf
can be used on any IO stream, including file handles and sockets.
scanf can be called either with or without a block.
scanf for Ruby scans an input string or stream according to a
<b>format</b>, as described below ("Conversions"), and returns an
array of matches between the format and the input. The format is
defined in a string, and is similar (though not identical) to the
formats used in Kernel#printf and Kernel#sprintf.
The format may contain <b>conversion specifiers</b>, which tell scanf
what form (type) each particular matched substring should be converted
to (e.g., decimal integer, floating point number, literal string,
etc.) The matches and conversions take place from left to right, and
the conversions themselves are returned as an array.
The format string may also contain characters other than those in the
conversion specifiers. White space (blanks, tabs, or newlines) in the
format string matches any amount of white space, including none, in
the input. Everything else matches only itself.
Scanning stops, and scanf returns, when any input character fails to
match the specifications in the format string, or when input is
exhausted, or when everything in the format string has been
matched. All matches found up to the stopping point are returned in
the return array (or yielded to the block, if a block was given).
==Basic usage
require 'scanf.rb'
# String#scanf and IO#scanf take a single argument (a format string)
array = aString.scanf("%d%s")
array = anIO.scanf("%d%s")
# Kernel#scanf reads from STDIN
array = scanf("%d%s")
==Block usage
When called with a block, scanf keeps scanning the input, cycling back
to the beginning of the format string, and yields a new array of
conversions to the block every time the format string is matched
(including partial matches, but not including complete failures). The
actual return value of scanf when called with a block is an array
containing the results of all the executions of the block.
str = "123 abc 456 def 789 ghi"
str.scanf("%d%s") { |num,str| [ num * 2, str.upcase ] }
# => [[246, "ABC"], [912, "DEF"], [1578, "GHI"]]
The single argument to scanf is a format string, which generally
includes one or more conversion specifiers. Conversion specifiers
begin with the percent character ('%') and include information about
what scanf should next scan for (string, decimal number, single
character, etc.).
There may be an optional maximum field width, expressed as a decimal
integer, between the % and the conversion. If no width is given, a
default of `infinity' is used (with the exception of the %c specifier;
see below). Otherwise, given a field width of <em>n</em> for a given
conversion, at most <em>n</em> characters are scanned in processing
that conversion. Before conversion begins, most conversions skip
white space in the input string; this white space is not counted
against the field width.
The following conversions are available. (See the files EXAMPLES
and <tt>tests/scanftests.rb</tt> for examples.)
Matches a literal `%'. That is, `%%' in the format string matches a
single input `%' character. No conversion is done, and the resulting
'%' is not included in the return array.
Matches an optionally signed decimal integer.
Same as d.
Matches an optionally signed integer. The integer is read in base
16 if it begins with `0x' or `0X', in base 8 if it begins with `0',
and in base 10 other- wise. Only characters that correspond to the
base are recognized.
Matches an optionally signed octal integer.
Matches an optionally signed hexadecimal integer,
Matches an optionally signed floating-point number.
Matches a sequence of non-white-space character. The input string stops at
white space or at the maximum field width, whichever occurs first.
Matches a single character, or a sequence of <em>n</em> characters if a
field width of <em>n</em> is specified. The usual skip of leading white
space is suppressed. To skip white space first, use an explicit space in
the format.
Matches a nonempty sequence of characters from the specified set
of accepted characters. The usual skip of leading white space is
suppressed. This bracketed sub-expression is interpreted exactly like a
character class in a Ruby regular expression. (In fact, it is placed as-is
in a regular expression.) The matching against the input string ends with
the appearance of a character not in (or, with a circumflex, in) the set,
or when the field width runs out, whichever comes first.
===Assignment suppression
To require that a particular match occur, but without including the result
in the return array, place the <b>assignment suppression flag</b>, which is
the star character ('*'), immediately after the leading '%' of a format
specifier (just before the field width, if any).
See the files <tt>EXAMPLES</tt> and <tt>tests/scanftests.rb</tt>.
==scanf for Ruby compared with scanf in C
scanf for Ruby is based on the C function scanf(3), but with modifications,
dictated mainly by the underlying differences between the languages.
===Unimplemented flags and specifiers
* The only flag implemented in scanf for Ruby is '<tt>*</tt>' (ignore
upcoming conversion). Many of the flags available in C versions of scanf(4)
have to do with the type of upcoming pointer arguments, and are literally
meaningless in Ruby.
* The <tt>n</tt> specifier (store number of characters consumed so far in
next pointer) is not implemented.
* The <tt>p</tt> specifier (match a pointer value) is not implemented.
===Altered specifiers
In scanf for Ruby, all of these specifiers scan for an optionally signed
integer, rather than for an unsigned integer like their C counterparts.
===Return values
scanf for Ruby returns an array of successful conversions, whereas
scanf(3) returns the number of conversions successfully
completed. (See below for more details on scanf for Ruby's return
==Return values
Without a block, scanf returns an array containing all the conversions
it has found. If none are found, scanf will return an empty array. An
unsuccesful match is never ignored, but rather always signals the end
of the scanning operation. If the first unsuccessful match takes place
after one or more successful matches have already taken place, the
returned array will contain the results of those successful matches.
With a block scanf returns a 'map'-like array of transformations from
the block -- that is, an array reflecting what the block did with each
yielded result from the iterative scanf operation. (See "Block
usage", above.)
==Test suite
scanf for Ruby includes a suite of unit tests (requiring the
<tt>TestUnit</tt> package), which can be run with the command <tt>ruby
tests/scanftests.rb</tt> or the command <tt>make test</tt>.
==Current limitations and bugs
When using IO#scanf under Windows, make sure you open your files in
binary mode:
File.open("filename", "rb")
so that scanf can keep track of characters correctly.
Support for character classes is reasonably complete (since it
essentially piggy-backs on Ruby's regular expression handling of
character classes), but users are advised that character class testing
has not been exhaustive, and that they should exercise some caution
in using any of the more complex and/or arcane character class
==Technical notes
===Rationale behind scanf for Ruby
The impetus for a scanf implementation in Ruby comes chiefly from the fact
that existing pattern matching operations, such as Regexp#match and
String#scan, return all results as strings, which have to be converted to
integers or floats explicitly in cases where what's ultimately wanted are
integer or float values.
===Design of scanf for Ruby
scanf for Ruby is essentially a <format string>-to-<regular
expression> converter.
When scanf is called, a FormatString object is generated from the
format string ("%d%s...") argument. The FormatString object breaks the
format string down into atoms ("%d", "%5f", "blah", etc.), and from
each atom it creates a FormatSpecifier object, which it
Each FormatSpecifier has a regular expression fragment and a "handler"
associated with it. For example, the regular expression fragment
associated with the format "%d" is "([-+]?\d+)", and the handler
associated with it is a wrapper around String#to_i. scanf itself calls
FormatString#match, passing in the input string. FormatString#match
iterates through its FormatSpecifiers; for each one, it matches the
corresponding regular expression fragment against the string. If
there's a match, it sends the matched string to the handler associated
with the FormatSpecifier.
Thus, to follow up the "%d" example: if "123" occurs in the input
string when a FormatSpecifier consisting of "%d" is reached, the "123"
will be matched against "([-+]?\d+)", and the matched string will be
rendered into an integer by a call to to_i.
The rendered match is then saved to an accumulator array, and the
input string is reduced to the post-match substring. Thus the string
is "eaten" from the left as the FormatSpecifiers are applied in
sequence. (This is done to a duplicate string; the original string is
not altered.)
As soon as a regular expression fragment fails to match the string, or
when the FormatString object runs out of FormatSpecifiers, scanning
stops and results accumulated so far are returned in an array.
==License and copyright
Copyright:: (c) 2002-2003 David Alan Black
License:: Distributed on the same licensing terms as Ruby itself
==Warranty disclaimer
This software is provided "as is" and without any express or implied
warranties, including, without limitation, the implied warranties of
merchantibility and fitness for a particular purpose.
==Credits and acknowledgements
scanf for Ruby was developed as the major activity of the Austin
Ruby Codefest (Austin, Texas, August 2002).
Principal author:: David Alan Black (mailto:dblack@superlink.net)
Co-author:: Hal Fulton (mailto:hal9000@hypermetrics.com)
Project contributors:: Nolan Darilek, Jason Johnston
Thanks to Hal Fulton for hosting the Codefest.
Thanks to Matz for suggestions about the class design.
Thanks to Gavin Sinclair for some feedback on the documentation.
The text for parts of this document, especially the Description and
Conversions sections, above, were adapted from the Linux Programmer's
Manual manpage for scanf(3), dated 1995-11-01.
==Bugs and bug reports
scanf for Ruby is based on something of an amalgam of C scanf
implementations and documentation, rather than on a single canonical
description. Suggestions for features and behaviors which appear in
other scanfs, and would be meaningful in Ruby, are welcome, as are
reports of suspicious behaviors and/or bugs. (Please see "Credits and
acknowledgements", above, for email addresses.)
module Scanf
class FormatSpecifier
attr_reader :re_string, :matched_string, :conversion, :matched
def skip; /^\s*%\*/.match(@spec_string); end
def extract_float(s); s.to_f if s &&! skip; end
def extract_decimal(s); s.to_i if s &&! skip; end
def extract_hex(s); s.hex if s &&! skip; end
def extract_octal(s); s.oct if s &&! skip; end
def extract_integer(s); Integer(s) if s &&! skip; end
def extract_plain(s); s unless skip; end
def nil_proc(s); nil; end
def to_s
def count_space?
def initialize(str)
@spec_string = str
h = '[A-Fa-f0-9]'
@re_string, @handler =
case @spec_string
# %[[:...:]]
when /%\*?(\[\[:[a-z]+:\]\])/
[ "(#{$1}+)", :extract_plain ]
# %5[[:...:]]
when /%\*?(\d+)(\[\[:[a-z]+:\]\])/
[ "(#{$2}{1,#{$1}})", :extract_plain ]
# %[...]
when /%\*?\[([^\]]*)\]/
yes = $1
if /^\^/.match(yes) then no = yes[1..-1] else no = '^' + yes end
[ "([#{yes}]+)(?=[#{no}]|\\z)", :extract_plain ]
# %5[...]
when /%\*?(\d+)\[([^\]]*)\]/
yes = $2
w = $1
[ "([#{yes}]{1,#{w}})", :extract_plain ]
# %i
when /%\*?i/
[ "([-+]?(?:(?:0[0-7]+)|(?:0[Xx]#{h}+)|(?:[1-9]\\d*)))", :extract_integer ]
# %5i
when /%\*?(\d+)i/
n = $1.to_i
s = "("
if n > 1 then s += "[1-9]\\d{1,#{n-1}}|" end
if n > 1 then s += "0[0-7]{1,#{n-1}}|" end
if n > 2 then s += "[-+]0[0-7]{1,#{n-2}}|" end
if n > 2 then s += "[-+][1-9]\\d{1,#{n-2}}|" end
if n > 2 then s += "0[Xx]#{h}{1,#{n-2}}|" end
if n > 3 then s += "[-+]0[Xx]#{h}{1,#{n-3}}|" end
s += "\\d"
s += ")"
[ s, :extract_integer ]
# %d, %u
when /%\*?[du]/
[ '([-+]?\d+)', :extract_decimal ]
# %5d, %5u
when /%\*?(\d+)[du]/
n = $1.to_i
s = "("
if n > 1 then s += "[-+]\\d{1,#{n-1}}|" end
s += "\\d{1,#{$1}})"
[ s, :extract_decimal ]
# %x
when /%\*?[Xx]/
[ "([-+]?(?:0[Xx])?#{h}+)", :extract_hex ]
# %5x
when /%\*?(\d+)[Xx]/
n = $1.to_i
s = "("
if n > 3 then s += "[-+]0[Xx]#{h}{1,#{n-3}}|" end
if n > 2 then s += "0[Xx]#{h}{1,#{n-2}}|" end
if n > 1 then s += "[-+]#{h}{1,#{n-1}}|" end
s += "#{h}{1,#{n}}"
s += ")"
[ s, :extract_hex ]
# %o
when /%\*?o/
[ '([-+]?[0-7]+)', :extract_octal ]
# %5o
when /%\*?(\d+)o/
[ "([-+][0-7]{1,#{$1.to_i-1}}|[0-7]{1,#{$1}})", :extract_octal ]
# %f
when /%\*?f/
[ '([-+]?((\d+(?>(?=[^\d.]|$)))|(\d*(\.(\d*([eE][-+]?\d+)?)))))', :extract_float ]
# %5f
when /%\*?(\d+)f/
[ "(\\S{1,#{$1}})", :extract_float ]
# %5s
when /%\*?(\d+)s/
[ "(\\S{1,#{$1}})", :extract_plain ]
# %s
when /%\*?s/
[ '(\S+)', :extract_plain ]
# %c
when /\s%\*?c/
[ "\\s*(.)", :extract_plain ]
# %c
when /%\*?c/
[ "(.)", :extract_plain ]
# %5c (whitespace issues are handled by the count_*_space? methods)
when /%\*?(\d+)c/
[ "(.{1,#{$1}})", :extract_plain ]
# %%
when /%%/
[ '(\s*%)', :nil_proc ]
# literal characters
[ "(#{Regexp.escape(@spec_string)})", :nil_proc ]
@re_string = '\A' + @re_string
def to_re
def match(str)
@matched = false
s = str.dup
s.sub!(/\A\s+/,'') unless count_space?
res = to_re.match(s)
if res
@conversion = send(@handler, res[1])
@matched_string = @conversion.to_s
@matched = true
def letter
@spec_string[/%\*?\d*([a-z\[])/, 1]
def width
w = @spec_string[/%\*?(\d+)/, 1]
w && w.to_i
def mid_match?
return false unless @matched
cc_no_width = letter == '[' &&! width
c_or_cc_width = (letter == 'c' || letter == '[') && width
width_left = c_or_cc_width && (matched_string.size < width)
return width_left || cc_no_width
class FormatString
attr_reader :string_left, :last_spec_tried,
:last_match_tried, :matched_count, :space
SPECIFIERS = 'diuXxofeEgsc'
# possible space, followed by...
# percent sign, followed by...
# another percent sign, or...
# optional assignment suppression flag
# optional maximum field width
# named character class, ...
# traditional character class, or...
# specifier letter.
# or miscellaneous characters
def initialize(str)
@specs = []
@i = 1
s = str.to_s
return unless /\S/.match(s)
@space = true if /\s\z/.match(s)
@specs.replace s.scan(REGEX).map {|spec| FormatSpecifier.new(spec) }
def to_s
def prune(n=matched_count)
n.times { @specs.shift }
def spec_count
def last_spec
@i == spec_count - 1
def match(str)
accum = []
@string_left = str
@matched_count = 0
@specs.each_with_index do |spec,i|
@last_spec_tried = spec
@last_match_tried = spec.match(@string_left)
break unless @last_match_tried
@matched_count += 1
accum << spec.conversion
@string_left = @last_match_tried.post_match
break if @string_left.empty?
return accum.compact
class IO
# The trick here is doing a match where you grab one *line*
# of input at a time. The linebreak may or may not occur
# at the boundary where the string matches a format specifier.
# And if it does, some rule about whitespace may or may not
# be in effect...
# That's why this is much more elaborate than the string
# version.
# For each line:
# Match succeeds (non-emptily)
# and the last attempted spec/string sub-match succeeded:
# could the last spec keep matching?
# yes: save interim results and continue (next line)
# The last attempted spec/string did not match:
# are we on the next-to-last spec in the string?
# yes:
# is fmt_string.string_left all spaces?
# yes: does current spec care about input space?
# yes: fatal failure
# no: save interim results and continue
# no: continue [this state could be analyzed further]
def scanf(str,&b)
return block_scanf(str,&b) if b
return [] unless str.size > 0
start_position = pos rescue 0
matched_so_far = 0
source_buffer = ""
result_buffer = []
final_result = []
fstr = Scanf::FormatString.new(str)
loop do
if eof || (tty? &&! fstr.match(source_buffer))
source_buffer << gets
current_match = fstr.match(source_buffer)
spec = fstr.last_spec_tried
if spec.matched
if spec.mid_match?
elsif (fstr.matched_count == fstr.spec_count - 1)
if /\A\s*\z/.match(fstr.string_left)
break if spec.count_space?
matched_so_far += source_buffer.size
matched_so_far -= source_buffer.size
break if fstr.last_spec
seek(start_position + matched_so_far, IO::SEEK_SET) rescue Errno::ESPIPE
soak_up_spaces if fstr.last_spec && fstr.space
return final_result
def soak_up_spaces
c = getc
ungetc(c) if c
until eof ||! c || /\S/.match(c.chr)
c = getc
ungetc(c) if (c && /\S/.match(c.chr))
def block_scanf(str)
final = []
# Sub-ideal, since another FS gets created in scanf.
# But used here to determine the number of specifiers.
fstr = Scanf::FormatString.new(str)
last_spec = fstr.last_spec
current = scanf(str)
break if current.empty?
end until eof || fstr.last_spec_tried == last_spec
return final
class String
def scanf(fstr,&b)
if b
fs =
if fstr.is_a? Scanf::FormatString
def block_scanf(fstr,&b)
fs = Scanf::FormatString.new(fstr)
str = self.dup
final = []
current = str.scanf(fs)
final.push(yield(current)) unless current.empty?
str = fs.string_left
end until current.empty? || str.empty?
return final
module Kernel
def scanf(fs,&b)