# 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. OCaml strings used to be modifiable in place, for instance via the `String.set` and `String.blit` functions. This use is nowadays only possible when the compiler is put in "unsafe-string" mode by giving the `-unsafe-string` command-line option. This compatibility mode makes the types `string` and `bytes` (see `Bytes.t`) interchangeable so that functions expecting byte sequences can also accept strings as arguments and modify them.

The distinction between `bytes` and `string` was introduced in OCaml 4.02, and the "unsafe-string" compatibility mode was the default until OCaml 4.05. Starting with 4.06, the compatibility mode is opt-in; we intend to remove the option in the future.

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 4.02.0
- Raises `Invalid_argument` if `n < 0` or `n >` `Sys.max_string_length`.

    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`.

## 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.

## 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 4.03.0 (4.05.0 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 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 4.04.0 (4.05.0 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 4.00.0

    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 4.02.0

    val trim : string -> string

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

- Since 4.00.0

    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 4.03.0 (4.05.0 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 4.03.0 (4.05.0 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 4.03.0 (4.05.0 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 4.03.0 (4.05.0 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 4.00.0

## 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

## Converting

    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 iterator.

- 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

## Deprecated functions

    val create : int -> bytes

Deprecated.This is a deprecated alias of `Bytes.create`/`BytesLabels.create`.

`create n` returns a fresh byte sequence of length `n`. The sequence is uninitialized and contains arbitrary bytes.

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

    val set : bytes -> int -> char -> unit

Deprecated.This is a deprecated alias of `Bytes.set`/`BytesLabels.set`.

`set s n c` modifies byte sequence `s` in place, replacing the byte at index `n` with `c`. You can also write `s.[n] <- c` instead of `set s n c`.

- Raises `Invalid_argument` if `n` is not a valid index in `s`.

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

`blit src src_pos dst dst_pos len` copies `len` bytes from the string `src`, starting at index `src_pos`, to byte sequence `dst`, starting at character number `dst_pos`.

- Raises `Invalid_argument` if `src_pos` and `len` do not designate a valid range of `src`, or if `dst_pos` and `len` do not designate a valid range of `dst`.

    val copy : string -> string

Deprecated.Because strings are immutable, it doesn't make much sense to make identical copies of them.

Return a copy of the given string.

    val fill : bytes -> int -> int -> char -> unit

Deprecated.This is a deprecated alias of `Bytes.fill`/`BytesLabels.fill`.

`fill s pos len c` modifies byte sequence `s` in place, replacing `len` bytes by `c`, starting at `pos`.

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

    val uppercase : string -> string

Deprecated.Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with all lowercase letters translated to uppercase, including accented letters of the ISO Latin-1 (8859-1) character set.

    val lowercase : string -> string

Deprecated.Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with all uppercase letters translated to lowercase, including accented letters of the ISO Latin-1 (8859-1) character set.

    val capitalize : string -> string

Deprecated.Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with the first character set to uppercase, using the ISO Latin-1 (8859-1) character set..

    val uncapitalize : string -> string

Deprecated.Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with the first character set to lowercase, using the ISO Latin-1 (8859-1) character set.