# Module String

    module String: sig .. end

String operations.

Given a string `s` of length `l`, we call character number in `s` the index of a character in `s`. Indexes start at `0`, and we will call a character number valid in `s` if it falls within the range `[0...l-1]`. 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]`. Note that character number `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 can be modified in place, for instance via the `String.set` and `String.blit` functions described below. This possibility should be used rarely and with much care, however, since both the OCaml compiler and most OCaml libraries share strings as if they were immutable, rather than copying them. In particular, string literals are shared: a single copy of the string is created at program loading time and returned by all evaluations of the string literal. Consider for example:

`# let f () = "foo";;
val f : unit -> string = <fun>
# (f ()).[0] <- 'b';;
- : unit = ()
# f ();;
- : string = "boo"
`

Likewise, many functions from the standard library can return string literals or one of their string arguments. Therefore, the returned strings must not be modified directly. If mutation is absolutely necessary, it should be performed on a fresh copy of the string, as produced by `String.copy`.

    val length : string -> int

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

    val get : string -> int -> char

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

Raise `Invalid_argument` if `n` not a valid character number in `s`.

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

`String.set s n c` modifies string `s` in place, replacing the character number `n` by `c`. You can also write `s.[n] <- c` instead of `String.set s n c`.

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

    val create : int -> string

`String.create n` returns a fresh string of length `n`. The string initially contains arbitrary characters.

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 copy : string -> string

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 : string -> int -> int -> char -> unit

`String.fill s start len c` modifies string `s` in place, replacing `len` characters by `c`, starting at `start`.

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

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

`String.blit src srcoff dst dstoff len` copies `len` characters from string `src`, starting at character number `srcoff`, to string `dst`, starting at character number `dstoff`. It works correctly even if `src` and `dst` are the same string, and the source and destination intervals overlap.

Raise `Invalid_argument` if `srcoff` and `len` do not designate a valid substring of `src`, or if `dstoff` and `len` do not designate a valid substring of `dst`.

    val concat : string -> string list -> string

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

    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` and stores the results in a new string that is returned.
Since 4.00.0

    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 no 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. If there is no special character in the argument, return the original string itself, not a copy. Its inverse function is Scanf.unescaped.

    val index : string -> char -> int

`String.index s c` returns the character number 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 character number 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 character number 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 character number 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

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

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

Return a copy of the argument, with the first character set to uppercase.

    val uncapitalize : string -> string

Return a copy of the argument, with the first character set to lowercase.

    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`.