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 |
           +---+---+---+---+     +-----+

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;

The type for strings.

let make: int => char => string;

make n c is a string of length n with each index holding the character c.

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).

  • since 4.02
let empty: string;

The empty string.

  • since 4.13
let length: string => int;

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

let get: string => int => char;

get s i is the character at index i in s. This is the same as writing s.[i].

let of_bytes: bytes => string;

Return a new string that contains the same bytes as the given byte sequence.

  • since 4.13
let to_bytes: string => bytes;

Return a new byte sequence that contains the same bytes as the given string.

  • since 4.13
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.

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

concat sep ss concatenates the list of strings ss, inserting the separator string sep between each.

let cat: string => string => string;

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

  • since 4.13

Predicates and comparisons

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)
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.

let starts_with: prefix:string => string => bool;

starts_with ~prefix s is true if and only if s starts with prefix.

  • since 4.13
let ends_with: suffix:string => string => bool;

ends_with ~suffix s is true if and only if s ends with suffix.

  • since 4.13
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.

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.

let contains: string => char => bool;

contains s c is String.contains_from s 0 c.

Extracting substrings

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.

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

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
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
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
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
let for_all: (char => bool) => string => bool;

for_all p s checks if all characters in s satisfy the predicate p.

  • since 4.13
let exists: (char => bool) => string => bool;

exists p s checks if at least one character of s satisfies the predicate p.

  • since 4.13
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
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).

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)
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)
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)
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

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]; ().

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

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.

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).

  • since 4.05
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.

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).

  • since 4.05
let index: string => char => int;

index s c is String.index_from s 0 c.

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

index_opt s c is String.index_from_opt s 0 c.

  • since 4.05
let rindex: string => char => int;

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

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

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

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

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

  • since 4.07
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

UTF-8

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.

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

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.

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

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.

let 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.

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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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