# Module String

    module String: sig .. end

Strings.

A string `s` of length `n` is an indexable and immutable sequence of `n` bytes. For historical reasons these bytes are referred to as characters.

The semantics of string functions is defined in terms of indices and positions. These are depicted and described as follows.

    positions  0   1   2   3   4    n-1    n
               +---+---+---+---+     +-----+
      indices  | 0 | 1 | 2 | 3 | ... | n-1 |
               +---+---+---+---+     +-----+

- An index `i` of `s` is an integer in the range [`0`;`n-1`]. It represents the `i`th byte (character) of `s` which can be accessed using the constant time string indexing operator `s.[i]`.
- A position `i` of `s` is an integer in the range [`0`;`n`]. It represents either the point at the beginning of the string, or the point between two indices, or the point at the end of the string. The `i`th byte index is between position `i` and `i+1`.

Two integers `start` and `len` are said to define a valid substring of `s` if `len >= 0` and `start`, `start+len` are positions of `s`.

Unicode text. Strings being arbitrary sequences of bytes, they can hold any kind of textual encoding. However the recommended encoding for storing Unicode text in OCaml strings is UTF-8. This is the encoding used by Unicode escapes in string literals. For example the string `"\u{1F42B}"` is the UTF-8 encoding of the Unicode character U+1F42B.

Past mutability. Before OCaml 4.02, strings used to be modifiable in place like `Bytes.t` mutable sequences of bytes. OCaml 4 had various compiler flags and configuration options to support the transition period from mutable to immutable strings. Those options are no longer available, and strings are now always immutable.

The labeled version of this module can be used as described in the `StdLabels` module.

## Strings

    type t = string

The type for strings.

    val make : int -> char -> string

`make n c` is a string of length `n` with each index holding the character `c`.

- Raises `Invalid_argument` if `n < 0` or `n >` `Sys.max_string_length`.

    val init : int -> (int -> char) -> string

`init n f` is a string of length `n` with index `i` holding the character `f i` (called in increasing index order).

- Since 4.02
- Raises `Invalid_argument` if `n < 0` or `n >` `Sys.max_string_length`.

    val empty : string

The empty string.

- Since 4.13

    val length : string -> int

`length s` is the length (number of bytes/characters) of `s`.

    val get : string -> int -> char

`get s i` is the character at index `i` in `s`. This is the same as writing `s.[i]`.

- Raises `Invalid_argument` if `i` not an index of `s`.

    val of_bytes : bytes -> string

Return a new string that contains the same bytes as the given byte sequence.

- Since 4.13

    val to_bytes : string -> bytes

Return a new byte sequence that contains the same bytes as the given string.

- Since 4.13

    val blit : string -> int -> bytes -> int -> int -> unit

Same as `Bytes.blit_string` which should be preferred.

## Concatenating

Note. The `(^)` binary operator concatenates two strings.

    val concat : string -> string list -> string

`concat sep ss` concatenates the list of strings `ss`, inserting the separator string `sep` between each.

- Raises `Invalid_argument` if the result is longer than `Sys.max_string_length` bytes.

    val cat : string -> string -> string

`cat s1 s2` concatenates s1 and s2 (`s1 ^ s2`).

- Since 4.13
- Raises `Invalid_argument` if the result is longer than `Sys.max_string_length` bytes.

## Predicates and comparisons

    val equal : t -> t -> bool

`equal s0 s1` is `true` if and only if `s0` and `s1` are character-wise equal.

- Since 4.03 (4.05 in StringLabels)

    val compare : t -> t -> int

`compare s0 s1` sorts `s0` and `s1` in lexicographical order. `compare` behaves like `compare` on strings but may be more efficient.

    val starts_with : prefix:string -> string -> bool

`starts_with` `~prefix s` is `true` if and only if `s` starts with `prefix`.

- Since 4.13

    val ends_with : suffix:string -> string -> bool

`ends_with` `~suffix s` is `true` if and only if `s` ends with `suffix`.

- Since 4.13

    val contains_from : string -> int -> char -> bool

`contains_from s start c` is `true` if and only if `c` appears in `s` after position `start`.

- Raises `Invalid_argument` if `start` is not a valid position in `s`.

    val rcontains_from : string -> int -> char -> bool

`rcontains_from s stop c` is `true` if and only if `c` appears in `s` before position `stop+1`.

- Raises `Invalid_argument` if `stop < 0` or `stop+1` is not a valid position in `s`.

    val contains : string -> char -> bool

`contains s c` is `String.contains_from` `s 0 c`.

## Extracting substrings

    val sub : string -> int -> int -> string

`sub s pos len` is a string of length `len`, containing the substring of `s` that starts at position `pos` and has length `len`.

- Raises `Invalid_argument` if `pos` and `len` do not designate a valid substring of `s`.

    val split_on_char : char -> string -> string list

`split_on_char sep s` is the list of all (possibly empty) substrings of `s` that are delimited by the character `sep`.

The function's result is specified by the following invariants:

- The list is not empty.
- Concatenating its elements using `sep` as a separator returns a string equal to the input (`concat (make 1 sep)
  (split_on_char sep s) = s`).
- No string in the result contains the `sep` character.

- Since 4.04 (4.05 in StringLabels)

## Transforming

    val map : (char -> char) -> string -> string

`map f s` is the string resulting from applying `f` to all the characters of `s` in increasing order.

- Since 4.00

    val mapi : (int -> char -> char) -> string -> string

`mapi f s` is like `String.map` but the index of the character is also passed to `f`.

- Since 4.02

    val fold_left : ('acc -> char -> 'acc) -> 'acc -> string -> 'acc

`fold_left f x s` computes `f (... (f (f x s.[0]) s.[1]) ...) s.[n-1]`, where `n` is the length of the string `s`.

- Since 4.13

    val fold_right : (char -> 'acc -> 'acc) -> string -> 'acc -> 'acc

`fold_right f s x` computes `f s.[0] (f s.[1] ( ... (f s.[n-1] x) ...))`, where `n` is the length of the string `s`.

- Since 4.13

    val for_all : (char -> bool) -> string -> bool

`for_all p s` checks if all characters in `s` satisfy the predicate `p`.

- Since 4.13

    val exists : (char -> bool) -> string -> bool

`exists p s` checks if at least one character of `s` satisfies the predicate `p`.

- Since 4.13

    val trim : string -> string

`trim s` is `s` without leading and trailing whitespace. Whitespace characters are: `' '`, `'\x0C'` (form feed), `'\n'`, `'\r'`, and `'\t'`.

- Since 4.00

    val escaped : string -> string

`escaped s` is `s` with special characters represented by escape sequences, following the lexical conventions of OCaml.

All characters outside the US-ASCII printable range [0x20;0x7E] are escaped, as well as backslash (0x2F) and double-quote (0x22).

The function `Scanf.unescaped` is a left inverse of `escaped`, i.e. `Scanf.unescaped (escaped s) = s` for any string `s` (unless `escaped s` fails).

- Raises `Invalid_argument` if the result is longer than `Sys.max_string_length` bytes.

    val uppercase_ascii : string -> string

`uppercase_ascii s` is `s` with all lowercase letters translated to uppercase, using the US-ASCII character set.

- Since 4.03 (4.05 in StringLabels)

    val lowercase_ascii : string -> string

`lowercase_ascii s` is `s` with all uppercase letters translated to lowercase, using the US-ASCII character set.

- Since 4.03 (4.05 in StringLabels)

    val capitalize_ascii : string -> string

`capitalize_ascii s` is `s` with the first character set to uppercase, using the US-ASCII character set.

- Since 4.03 (4.05 in StringLabels)

    val uncapitalize_ascii : string -> string

`uncapitalize_ascii s` is `s` with the first character set to lowercase, using the US-ASCII character set.

- Since 4.03 (4.05 in StringLabels)

## Traversing

    val iter : (char -> unit) -> string -> unit

`iter f s` applies function `f` in turn to all the characters of `s`. It is equivalent to `f s.[0]; f s.[1]; ...; f s.[length s - 1]; ()`.

    val iteri : (int -> char -> unit) -> string -> unit

`iteri` is like `String.iter`, but the function is also given the corresponding character index.

- Since 4.00

## Searching

    val index_from : string -> int -> char -> int

`index_from s i c` is the index of the first occurrence of `c` in `s` after position `i`.

- Raises
  - `Not_found` if `c` does not occur in `s` after position `i`.
  - `Invalid_argument` if `i` is not a valid position in `s`.

    val index_from_opt : string -> int -> char -> int option

`index_from_opt s i c` is the index of the first occurrence of `c` in `s` after position `i` (if any).

- Since 4.05
- Raises `Invalid_argument` if `i` is not a valid position in `s`.

    val rindex_from : string -> int -> char -> int

`rindex_from s i c` is the index of the last occurrence of `c` in `s` before position `i+1`.

- Raises
  - `Not_found` if `c` does not occur in `s` before position `i+1`.
  - `Invalid_argument` if `i+1` is not a valid position in `s`.

    val rindex_from_opt : string -> int -> char -> int option

`rindex_from_opt s i c` is the index of the last occurrence of `c` in `s` before position `i+1` (if any).

- Since 4.05
- Raises `Invalid_argument` if `i+1` is not a valid position in `s`.

    val index : string -> char -> int

`index s c` is `String.index_from` `s 0 c`.

    val index_opt : string -> char -> int option

`index_opt s c` is `String.index_from_opt` `s 0 c`.

- Since 4.05

    val rindex : string -> char -> int

`rindex s c` is `String.rindex_from` `s (length s - 1) c`.

    val rindex_opt : string -> char -> int option

`rindex_opt s c` is `String.rindex_from_opt` `s (length s - 1) c`.

- Since 4.05

## Strings and Sequences

    val to_seq : t -> char Seq.t

`to_seq s` is a sequence made of the string's characters in increasing order. In `"unsafe-string"` mode, modifications of the string during iteration will be reflected in the sequence.

- Since 4.07

    val to_seqi : t -> (int * char) Seq.t

`to_seqi s` is like `String.to_seq` but also tuples the corresponding index.

- Since 4.07

    val of_seq : char Seq.t -> t

`of_seq s` is a string made of the sequence's characters.

- Since 4.07

## UTF decoding and validations

### UTF-8

    val get_utf_8_uchar : t -> int -> Uchar.utf_decode

`get_utf_8_uchar b i` decodes an UTF-8 character at index `i` in `b`.

    val is_valid_utf_8 : t -> bool

`is_valid_utf_8 b` is `true` if and only if `b` contains valid UTF-8 data.

### UTF-16BE

    val get_utf_16be_uchar : t -> int -> Uchar.utf_decode

`get_utf_16be_uchar b i` decodes an UTF-16BE character at index `i` in `b`.

    val is_valid_utf_16be : t -> bool

`is_valid_utf_16be b` is `true` if and only if `b` contains valid UTF-16BE data.

### UTF-16LE

    val get_utf_16le_uchar : t -> int -> Uchar.utf_decode

`get_utf_16le_uchar b i` decodes an UTF-16LE character at index `i` in `b`.

    val is_valid_utf_16le : t -> bool

`is_valid_utf_16le b` is `true` if and only if `b` contains valid UTF-16LE data.

## Binary decoding of integers

The functions in this section binary decode integers from strings.

All following functions raise `Invalid_argument` if the characters needed at index `i` to decode the integer are not available.

Little-endian (resp. big-endian) encoding means that least (resp. most) significant bytes are stored first. Big-endian is also known as network byte order. Native-endian encoding is either little-endian or big-endian depending on `Sys.big_endian`.

32-bit and 64-bit integers are represented by the `int32` and `int64` types, which can be interpreted either as signed or unsigned numbers.

8-bit and 16-bit integers are represented by the `int` type, which has more bits than the binary encoding. These extra bits are sign-extended (or zero-extended) for functions which decode 8-bit or 16-bit integers and represented them with `int` values.

    val get_uint8 : string -> int -> int

`get_uint8 b i` is `b`'s unsigned 8-bit integer starting at character index `i`.

- Since 4.13

    val get_int8 : string -> int -> int

`get_int8 b i` is `b`'s signed 8-bit integer starting at character index `i`.

- Since 4.13

    val get_uint16_ne : string -> int -> int

`get_uint16_ne b i` is `b`'s native-endian unsigned 16-bit integer starting at character index `i`.

- Since 4.13

    val get_uint16_be : string -> int -> int

`get_uint16_be b i` is `b`'s big-endian unsigned 16-bit integer starting at character index `i`.

- Since 4.13

    val get_uint16_le : string -> int -> int

`get_uint16_le b i` is `b`'s little-endian unsigned 16-bit integer starting at character index `i`.

- Since 4.13

    val get_int16_ne : string -> int -> int

`get_int16_ne b i` is `b`'s native-endian signed 16-bit integer starting at character index `i`.

- Since 4.13

    val get_int16_be : string -> int -> int

`get_int16_be b i` is `b`'s big-endian signed 16-bit integer starting at character index `i`.

- Since 4.13

    val get_int16_le : string -> int -> int

`get_int16_le b i` is `b`'s little-endian signed 16-bit integer starting at character index `i`.

- Since 4.13

    val get_int32_ne : string -> int -> int32

`get_int32_ne b i` is `b`'s native-endian 32-bit integer starting at character index `i`.

- Since 4.13

    val hash : t -> int

An unseeded hash function for strings, with the same output value as `Hashtbl.hash`. This function allows this module to be passed as argument to the functor `Hashtbl.Make`.

- Since 5.0

    val seeded_hash : int -> t -> int

A seeded hash function for strings, with the same output value as `Hashtbl.seeded_hash`. This function allows this module to be passed as argument to the functor `Hashtbl.MakeSeeded`.

- Since 5.0

    val get_int32_be : string -> int -> int32

`get_int32_be b i` is `b`'s big-endian 32-bit integer starting at character index `i`.

- Since 4.13

    val get_int32_le : string -> int -> int32

`get_int32_le b i` is `b`'s little-endian 32-bit integer starting at character index `i`.

- Since 4.13

    val get_int64_ne : string -> int -> int64

`get_int64_ne b i` is `b`'s native-endian 64-bit integer starting at character index `i`.

- Since 4.13

    val get_int64_be : string -> int -> int64

`get_int64_be b i` is `b`'s big-endian 64-bit integer starting at character index `i`.

- Since 4.13

    val get_int64_le : string -> int -> int64

`get_int64_le b i` is `b`'s little-endian 64-bit integer starting at character index `i`.

- Since 4.13