# Module String

    module String: sig .. end

String operations.

A string is an immutable data structure that contains a fixed-length sequence of (single-byte) characters. Each character can be accessed in constant time through its index.

Given a string `s` of length `l`, we can access each of the `l` characters of `s` via its index in the sequence. Indexes start at `0`, and we will call an index valid in `s` if it falls within the range `[0...l-1]` (inclusive). A position is the point between two characters or at the beginning or end of the string. We call a position valid in `s` if it falls within the range `[0...l]` (inclusive). Note that the character at index `n` is between positions `n` and `n+1`.

Two parameters `start` and `len` are said to designate a valid substring of `s` if `len >= 0` and `start` and `start+len` are valid positions in `s`.

OCaml strings used to be modifiable in place, for instance via the `String.set` and `String.blit` functions described below. This usage is deprecated and only possible when the compiler is put in "unsafe-string" mode by giving the `-unsafe-string` command-line option (which is currently the default for reasons of backward compatibility). This is done by making the types `string` and `bytes` (see module `Bytes`) interchangeable so that functions expecting byte sequences can also accept strings as arguments and modify them.

All new code should avoid this feature and be compiled with the `-safe-string` command-line option to enforce the separation between the types `string` and `bytes`.

    val length : string -> int

Return the length (number of characters) of the given string.

    val get : string -> int -> char

`String.get s n` returns the character at index `n` in string `s`. You can also write `s.[n]` instead of `String.get s n`.

Raise `Invalid_argument` if `n` not a valid index in `s`.

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

Deprecated.This is a deprecated alias of `Bytes.set`.``
`String.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 `String.set s n c`.

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

    val create : int -> bytes

Deprecated.This is a deprecated alias of `Bytes.create`.``
`String.create n` returns a fresh byte sequence of length `n`. The sequence is uninitialized and contains arbitrary bytes.

Raise `Invalid_argument` if `n < 0` or `n >` `Sys.max_string_length`.

    val make : int -> char -> string

`String.make n c` returns a fresh string of length `n`, filled with the character `c`.

Raise `Invalid_argument` if `n < 0` or `n >` `Sys.max_string_length`.

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

`String.init n f` returns a string of length `n`, with character `i` initialized to the result of `f i` (called in increasing index order).

Raise `Invalid_argument` if `n < 0` or `n >` `Sys.max_string_length`.
Since 4.02.0

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

`String.sub s start len` returns a fresh string of length `len`, containing the substring of `s` that starts at position `start` and has length `len`.

Raise `Invalid_argument` if `start` and `len` do not designate a valid substring of `s`.

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

Deprecated.This is a deprecated alias of `Bytes.fill`.``
`String.fill s start len c` modifies byte sequence `s` in place, replacing `len` bytes with `c`, starting at `start`.

Raise `Invalid_argument` if `start` and `len` do not designate a valid range of `s`.

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

Same as `Bytes.blit_string`.

    val concat : string -> string list -> string

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

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

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

`String.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.[String.length s - 1]; ()`.

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

Same as `String.iter`, but the function is applied to the index of the element as first argument (counting from 0), and the character itself as second argument.
Since 4.00.0

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

`String.map f s` applies function `f` in turn to all the characters of `s` (in increasing index order) and stores the results in a new string that is returned.
Since 4.00.0

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

`String.mapi f s` calls `f` with each character of `s` and its index (in increasing index order) and stores the results in a new string that is returned.
Since 4.02.0

    val trim : string -> string

Return a copy of the argument, without leading and trailing whitespace. The characters regarded as whitespace are: `' '`, `'\012'`, `'\n'`, `'\r'`, and `'\t'`. If there is neither leading nor trailing whitespace character in the argument, return the original string itself, not a copy.
Since 4.00.0

    val escaped : string -> string

Return a copy of the argument, with special characters represented by escape sequences, following the lexical conventions of OCaml. All characters outside the ASCII printable range (32..126) are escaped, as well as backslash and double-quote.

If there is no special character in the argument that needs escaping, return the original string itself, not a copy.

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

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

    val index : string -> char -> int

`String.index s c` returns the index of the first occurrence of character `c` in string `s`.

Raise `Not_found` if `c` does not occur in `s`.

    val rindex : string -> char -> int

`String.rindex s c` returns the index of the last occurrence of character `c` in string `s`.

Raise `Not_found` if `c` does not occur in `s`.

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

`String.index_from s i c` returns the index of the first occurrence of character `c` in string `s` after position `i`. `String.index s c` is equivalent to `String.index_from s 0 c`.

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

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

`String.rindex_from s i c` returns the index of the last occurrence of character `c` in string `s` before position `i+1`. `String.rindex s c` is equivalent to `String.rindex_from s (String.length s - 1) c`.

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

    val contains : string -> char -> bool

`String.contains s c` tests if character `c` appears in the string `s`.

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

`String.contains_from s start c` tests if character `c` appears in `s` after position `start`. `String.contains s c` is equivalent to `String.contains_from s 0 c`.

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

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

`String.rcontains_from s stop c` tests if character `c` appears in `s` before position `stop+1`.

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

    val uppercase_ascii : string -> string

Return a copy of the argument, with all lowercase letters translated to uppercase, using the US-ASCII character set.
Since 4.03.0

    val lowercase_ascii : string -> string

Return a copy of the argument, with all uppercase letters translated to lowercase, using the US-ASCII character set.
Since 4.03.0

    val capitalize_ascii : string -> string

Return a copy of the argument, with the first character set to uppercase, using the US-ASCII character set.
Since 4.03.0

    val uncapitalize_ascii : string -> string

Return a copy of the argument, with the first character set to lowercase, using the US-ASCII character set.
Since 4.03.0

    type t = string

An alias for the type of strings.

    val compare : t -> t -> int

The comparison function for strings, with the same specification as `compare`. Along with the type `t`, this function `compare` allows the module `String` to be passed as argument to the functors `Set.Make` and `Map.Make`.

    val equal : t -> t -> bool

The equal function for strings.
Since 4.03.0

    val split_on_char : char -> string -> string list

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

The function's output 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 (`String.concat (String.make 1 sep)
  (String.split_on_char sep s) = s`).
- No string in the result contains the `sep` character.

Since 4.04.0