Scanf

As an example, consider the formatted input function scanf that reads from standard input; a typical call to scanf is simply scanf fmt f, meaning that f should be applied to the arguments read according to the format string fmt. For instance, if f is defined as let f x = x + 1, then scanf "%d" f will read a decimal integer i from stdin and return f i; thus, if we enter 41 at the keyboard, scanf "%d" f evaluates to 42.

This module provides general formatted input functions that read from any kind of input, including strings, files, or anything that can return characters. Hence, a typical call to a formatted input function bscan is bscan ib fmt f, meaning that f should be applied to the arguments read from input ib, according to the format string fmt.

The Caml scanning facility is reminiscent of the corresponding C feature. However, it is also largely different, simpler, and yet more powerful: the formatted input functions are higher-order functionals and the parameter passing mechanism is simply the regular function application not the variable assigment based mechanism which is typical of formatted input in imperative languages; the format strings also feature useful additions to easily define complex tokens; as expected of a functional programming language feature, the formatted input functions support polymorphism, in particular arbitrary interaction with polymorphic user-defined scanners. Furthermore, the Caml formatted input facility is fully type-checked at compile time.

The exception raised by formatted input functions when the input cannot be read according to the given format.

The type of formatted input scanners: ('a, 'b, 'c, 'd) scanner is the type of a formatted input function that reads from some scanning buffer according to some format string; more precisely, if scan is some formatted input function, then scan ib fmt f applies f to the arguments specified by the format string fmt, when scan has read those arguments from some scanning buffer ib.

For instance, the scanf function below has type ('a, 'b, 'c, 'd) scanner, since it is a formatted input function that reads from stdib: scanf fmt f applies f to the arguments specified by fmt, reading those arguments from stdin as expected.

If the format fmt has some %r indications, the corresponding input functions must be provided before the f argument. For instance, if read_elem is an input function for values of type t, then bscanf ib "%r;" read_elem f reads a value of type t followed by a ';' character.

Formatted input functions

bscanf ib fmt r1 ... rN f reads arguments for the function f from the scanning buffer ib according to the format string fmt, and applies f to these values. The result of this call to f is returned as the result of bscanf. For instance, if f is the function fun s i -> i + 1, then Scanf.sscanf "x = 1" "%s = %i" f returns 2.

Arguments r1 to rN are user-defined input functions that read the argument corresponding to a %r conversion.

The format is a character string which contains three types of objects:

plain characters, which are simply matched with the characters of the input,
conversion specifications, each of which causes reading and conversion of one argument for f,
scanning indications to specify boundaries of tokens.

Among plain characters the space character (ASCII code 32) has a special meaning: it matches ``whitespace'', that is any number of tab, space, line feed and carriage return characters. Hence, a space in the format matches any amount of whitespace in the input.

Conversion specifications consist in the % character, followed by an optional flag, an optional field width, and followed by one or two conversion characters. The conversion characters and their meanings are:

d: reads an optionally signed decimal integer.
i: reads an optionally signed integer (usual input formats for hexadecimal (0x[d]+ and 0X[d]+), octal (0o[d]+), and binary 0b[d]+ notations are understood).
u: reads an unsigned decimal integer.
x or X: reads an unsigned hexadecimal integer.
o: reads an unsigned octal integer.
s: reads a string argument that spreads as much as possible, until the next white space, the next scanning indication, or the end-of-input is reached. Hence, this conversion always succeeds: it returns an empty string if the bounding condition holds when the scan begins.
S: reads a delimited string argument (delimiters and special escaped characters follow the lexical conventions of Caml).
c: reads a single character. To test the current input character without reading it, specify a null field width, i.e. use specification %0c. Raise Invalid_argument, if the field width specification is greater than 1.
C: reads a single delimited character (delimiters and special escaped characters follow the lexical conventions of Caml).
f, e, E, g, G: reads an optionally signed floating-point number in decimal notation, in the style dddd.ddd e/E+-dd.
F: reads a floating point number according to the lexical conventions of Caml (hence the decimal point is mandatory if the exponent part is not mentioned).
B: reads a boolean argument (true or false).
b: reads a boolean argument (for backward compatibility; do not use in new programs).
ld, li, lu, lx, lX, lo: reads an int32 argument to the format specified by the second letter (decimal, hexadecimal, etc).
nd, ni, nu, nx, nX, no: reads a nativeint argument to the format specified by the second letter.
Ld, Li, Lu, Lx, LX, Lo: reads an int64 argument to the format specified by the second letter.
[ range ]: reads characters that matches one of the characters mentioned in the range of characters range (or not mentioned in it, if the range starts with ^). Reads a string that can be empty, if the next input character does not match the range. The set of characters from c1 to c2 (inclusively) is denoted by c1-c2. Hence, %[0-9] returns a string representing a decimal number or an empty string if no decimal digit is found; similarly, %[\\048-\\057\\065-\\070] returns a string of hexadecimal digits. If a closing bracket appears in a range, it must occur as the first character of the range (or just after the ^ in case of range negation); hence []] matches a ] character and [^]] matches any character that is not ].
r: user-defined reader. Takes the next ri formatted input function and applies it to the scanning buffer ib to read the next argument. The input function ri must therefore have type Scanning.scanbuf -> 'a and the argument read has type 'a.
{ fmt %}: reads a format string argument to the format specified by the internal format fmt. The format string to be read must have the same type as the internal format fmt. For instance, "%{%i%}" reads any format string that can read a value of type int; hence Scanf.sscanf "fmt:\\\"number is %u\\\"" "fmt:%{%i%}" succeeds and returns the format string "number is %u".
\( fmt %\): scanning format substitution. Reads a format string to replace fmt. The format string read must have the same type as fmt.
l: returns the number of lines read so far.
n: returns the number of characters read so far.
N or L: returns the number of tokens read so far.
!: matches the end of input condition.
%: matches one % character in the input.

Following the % character that introduces a conversion, there may be the special flag _: the conversion that follows occurs as usual, but the resulting value is discarded. For instance, if f is the function fun i -> i + 1, then Scanf.sscanf "x = 1" "%_s = %i" f returns 2.

The field width is composed of an optional integer literal indicating the maximal width of the token to read. For instance, %6d reads an integer, having at most 6 decimal digits; %4f reads a float with at most 4 characters; and %8[\\000-\\255] returns the next 8 characters (or all the characters still available, if fewer than 8 characters are available in the input).

Scanning indications appear just after the string conversions %s and %[ range ] to delimit the end of the token. A scanning indication is introduced by a @ character, followed by some constant character c. It means that the string token should end just before the next matching c (which is skipped). If no c character is encountered, the string token spreads as much as possible. For instance, "%s@\t" reads a string up to the next tab character or to the end of input. If a scanning indication @c does not follow a string conversion, it is treated as a plain c character.

Raise Scanf.Scan_failure if the input does not match the format.

Raise Failure if a conversion to a number is not possible.

Raise End_of_file if the end of input is encountered while some more characters are needed to read the current conversion specification. As a consequence, scanning a %s conversion never raises exception End_of_file: if the end of input is reached the conversion succeeds and simply returns the characters read so far, or "" if none were read.

Raise Invalid_argument if the format string is invalid.

Notes:

the scanning indications introduce slight differences in the syntax of Scanf format strings compared to those used by the Printf module. However, scanning indications are similar to those of the Format module; hence, when producing formatted text to be scanned by !Scanf.bscanf, it is wise to use printing functions from Format (or, if you need to use functions from Printf, banish or carefully double check the format strings that contain '@' characters).

in addition to relevant digits, '_' characters may appear inside numbers (this is reminiscent to the usual Caml lexical conventions). If stricter scanning is desired, use the range conversion facility instead of the number conversions.

the scanf facility is not intended for heavy duty lexical analysis and parsing. If it appears not expressive enough for your needs, several alternative exists: regular expressions (module Str), stream parsers, ocamllex-generated lexers, ocamlyacc-generated parsers.

Module Scanf

Functional input with format strings.

Formatted input functions