# Module Stdlib.String

    module String: String

## 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