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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 format, 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 conversion specifiers, 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 n for a given conversion, at most n 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 tests/scanftests.rb 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 n characters if a field width of n 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 assignment suppression flag, which is the star character ('*'), immediately after the leading '%' of a format specifier (just before the field width, if any).


See the files EXAMPLES and tests/scanftests.rb.

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 '*' (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 n specifier (store number of characters consumed so far in next pointer) is not implemented.

  • The p 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 values.)

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 TestUnit package), which can be run with the command ruby tests/scanftests.rb or the command make test.

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 idioms.

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 saves.

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 Scanf::FormatString#match, passing in the input string. Scanf::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


© 2002-2003 David Alan Black


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 (dblack@superlink.net)


Hal Fulton (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.)