# Module Scanf

    module Scanf: sig .. end

Formatted input functions.

    module Scanning: sig .. end

Scanning buffers.

    exception Scan_failure of string

The exception that formatted input functions raise when the input cannot be read according to the given format.

    val bscanf : Scanning.scanbuf ->
           ('a, Scanning.scanbuf, 'b) format -> 'a -> 'b

`bscanf ib fmt f` reads tokens from the scanning buffer `ib` according to the format string `fmt`, converts these tokens to values, and applies the function `f` to these values. The result of this application of `f` is the result of the whole construct.

For instance, if `p` is the function `fun s i -> i + 1`, then `Scanf.sscanf "x = 1" "%s = %i" p` returns `2`.

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, newline 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 no character in the input matches 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 `]`.
- `{ 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`: applies `f` to the number of lines read so far.
- `n`: applies `f` to the number of characters read so far.
- `N` or `L`: applies `f` to the number of tokens read so far.
- `!`: matches the end of input condition.
- `%`: matches one `%` character in the input.

Following the `%` character introducing a conversion, there may be the special flag `_`: the conversion that follows occurs as usual, but the resulting value is discarded.

The field widths are 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 less 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 tabulation 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 given 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 (this means in particular that scanning a `%s` conversion never raises exception `End_of_file`: if the end of input is reached the conversion succeeds and simply returns `""`).

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 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.

    val fscanf : in_channel ->
           ('a, Scanning.scanbuf, 'b) format -> 'a -> 'b

Same as `Scanf.bscanf`, but inputs from the given channel.

Warning: since all scanning functions operate from a scanning buffer, be aware that each `fscanf` invocation must allocate a new fresh scanning buffer (unless careful use of partial evaluation in the program). Hence, there are chances that some characters seem to be skipped (in fact they are pending in the previously used buffer). This happens in particular when calling `fscanf` again after a scan involving a format that necessitates some look ahead (such as a format that ends by skipping whitespace in the input).

To avoid confusion, consider using `bscanf` with an explicitly created scanning buffer. Use for instance `Scanning.from_file f` to allocate the scanning buffer reading from file `f`.

This method is not only clearer it is also faster, since scanning buffers to files are optimized for fast bufferized reading.

    val sscanf : string -> ('a, Scanning.scanbuf, 'b) format -> 'a -> 'b

Same as `Scanf.bscanf`, but inputs from the given string.

    val scanf : ('a, Scanning.scanbuf, 'b) format -> 'a -> 'b

Same as `Scanf.bscanf`, but reads from the predefined scanning buffer `Scanf.Scanning.stdib` that is connected to `stdin`.

    val kscanf : Scanning.scanbuf ->
           (Scanning.scanbuf -> exn -> 'a) ->
           ('b, Scanning.scanbuf, 'a) format -> 'b -> 'a

Same as `Scanf.bscanf`, but takes an additional function argument `ef` that is called in case of error: if the scanning process or some conversion fails, the scanning function aborts and applies the error handling function `ef` to the scanning buffer and the exception that aborted the scanning process.

    val bscanf_format : Scanning.scanbuf ->
           ('a, 'b, 'c, 'd) format4 -> (('a, 'b, 'c, 'd) format4 -> 'e) -> 'e

`bscanf_format ib fmt f` reads a `format` argument to the format specified by the second argument. The `format` argument read in buffer `ib` must have the same type as `fmt`.

    val sscanf_format : string -> ('a, 'b, 'c, 'd) format4 -> ('a, 'b, 'c, 'd) format4

`sscanf_format ib fmt f` reads a `format` argument to the format specified by the second argument and returns it. The `format` argument read in string `s` must have the same type as `fmt`.