Module Stdlib.String

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 ith 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 ith 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

type t = string;

The type for strings.

val make : int -> char -> string

let 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

let 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).

raises Invalid_argument if n < 0 or n > Sys.max_string_length. since 4.02

val empty : string

let empty: string;

The empty string.

since 4.13

val length : string -> int

let length: string => int;

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

val get : string -> int -> char

let 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

let 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

let 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

let blit: string => int => bytes => int => int => unit;

Same as Bytes.blit_string which should be preferred.

Concatenating

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

val concat : string -> string list -> string

let concat: string => list(string) => 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

let cat: string => string => string;

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

raises Invalid_argument if the result is longer than Sys.max_string_length bytes. since 4.13

Predicates and comparisons

val equal : t -> t -> bool

let 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

let compare: t => t => int;

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

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

let 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

let 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

let 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

let 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

let contains: string => char => bool;

contains s c is String.contains_from s 0 c.

Extracting substrings

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

let 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

let split_on_char: char => string => list(string);

split_on_char sep s is the list of all (possibly empty) substrings of s that are delimited by the character sep. If s is empty, the result is the singleton list [""].

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

let 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

let mapi: (int => char => char) => string => string;

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

since 4.02

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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let iteri: (int => char => unit) => string => unit;

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

since 4.00

Searching

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

let 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. raises Invalid_argument if i is not a valid position in s.

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

let index_from_opt: string => int => char => option(int);

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

raises Invalid_argument if i is not a valid position in s. since 4.05

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

let 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. raises Invalid_argument if i+1 is not a valid position in s.

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

let rindex_from_opt: string => int => char => option(int);

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

raises Invalid_argument if i+1 is not a valid position in s. since 4.05

val index : string -> char -> int

let index: string => char => int;

index s c is String.index_from s 0 c.

val index_opt : string -> char -> int option

let index_opt: string => char => option(int);

index_opt s c is String.index_from_opt s 0 c.

since 4.05

val rindex : string -> char -> int

let rindex: string => char => int;

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

val rindex_opt : string -> char -> int option

let rindex_opt: string => char => option(int);

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

let to_seq: t => Seq.t(char);

to_seq s is a sequence made of the string's characters in increasing order.

since 4.07

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

let to_seqi: t => Seq.t((int, char));

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

since 4.07

val of_seq : char Seq.t -> t

let of_seq: Seq.t(char) => t;

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

since 4.07

UTF decoding and validations

since 4.14

UTF-8

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

let 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

let 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

let 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

let 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

let 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

let is_valid_utf_16le: t => bool;

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

Spellchecking

val edit_distance : ?limit:int -> t -> t -> int

let edit_distance: ?limit:int => t => t => int;

edit_distance s0 s1 is the number of single character edits (understood as insertion, deletion, substitution, transposition) that are needed to change s0 into s1.

If limit is provided the function returns with limit as soon as it was determined that s0 and s1 have distance of at least limit. This is faster if you have a fixed limit, for example for spellchecking.

The function assumes the strings are UTF-8 encoded and uses Uchar.t for the notion of character. Decoding errors are replaced by Uchar.rep. Normalizing the strings to NFC gives better results.

Note. This implements the simpler Optimal String Alignement (OSA) distance, not the Damerau-Levenshtein distance. With this function "ca" and "abc" have a distance of 3 not 2.

since 5.4

val spellcheck : 
  ?max_dist:(string -> int) ->
  ((string -> unit) -> unit) ->
  string ->
  string list

let spellcheck: 
  ?max_dist:(string => int) =>
  ((string => unit) => unit) =>
  string =>
  list(string);

spellcheck iter_dict s are the strings enumerated by the iterator iter_dict whose edit distance to s is the smallest and at most max_dist s. If multiple corrections are returned their order is as found in iter_dict. The default max_dist s is:

• 0 if s has 0 to 2 Unicode characters.
• 1 if s has 3 to 4 Unicode characters.
• 2 otherwise. If your dictionary is a list l, a suitable iter_dict is given by (fun yield -> List.iter yield l).

All strings are assumed to be UTF-8 encoded, decoding errors are replaced by Uchar.rep characters.

since 5.4

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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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

let 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