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
val make : int -> char -> stringmake n c is a string of length n with each index holding the character c.
val init : int -> f:(int -> char) -> stringinit n ~f is a string of length n with index i holding the character f i (called in increasing index order).
val of_bytes : bytes -> stringReturn a new string that contains the same bytes as the given byte sequence.
val to_bytes : string -> bytesReturn a new byte sequence that contains the same bytes as the given string.
val length : string -> intlength s is the length (number of bytes/characters) of s.
val get : string -> int -> charget s i is the character at index i in s. This is the same as writing s.[i].
Concatenating
Note. The Stdlib.(^) binary operator concatenates two strings.
val concat : sep:string -> string list -> stringconcat ~sep ss concatenates the list of strings ss, inserting the separator string sep between each.
val cat : string -> string -> stringcat s1 s2 concatenates s1 and s2 (s1 ^ s2).
Predicates and comparisons
val equal : t -> t -> boolequal s0 s1 is true if and only if s0 and s1 are character-wise equal.
val compare : t -> t -> intcompare 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 -> boolstarts_with ~prefix s is true if and only if s starts with prefix.
val ends_with : suffix:string -> string -> boolends_with ~suffix s is true if and only if s ends with suffix.
val contains_from : string -> int -> char -> boolcontains_from s start c is true if and only if c appears in s after position start.
val rcontains_from : string -> int -> char -> boolrcontains_from s stop c is true if and only if c appears in s before position stop+1.
val contains : string -> char -> boolval sub : string -> pos:int -> len:int -> stringsub s ~pos ~len is a string of length len, containing the substring of s that starts at position pos and has length len.
val split_on_char : sep:char -> string -> string listsplit_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.
val map : f:(char -> char) -> string -> stringmap f s is the string resulting from applying f to all the characters of s in increasing order.
val mapi : f:(int -> char -> char) -> string -> stringmapi ~f s is like map but the index of the character is also passed to f.
val fold_left : f:('a -> char -> 'a) -> init:'a -> string -> 'afold_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.
val fold_right : f:(char -> 'a -> 'a) -> string -> init:'a -> 'afold_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.
val for_all : f:(char -> bool) -> string -> boolfor_all p s checks if all characters in s satisfy the predicate p.
val exists : f:(char -> bool) -> string -> boolexists p s checks if at least one character of s satisfies the predicate p.
val trim : string -> stringtrim s is s without leading and trailing whitespace. Whitespace characters are: ' ', '\x0C' (form feed), '\n', '\r', and '\t'.
val escaped : string -> stringescaped 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).
val uppercase_ascii : string -> stringuppercase_ascii s is s with all lowercase letters translated to uppercase, using the US-ASCII character set.
val lowercase_ascii : string -> stringlowercase_ascii s is s with all uppercase letters translated to lowercase, using the US-ASCII character set.
val capitalize_ascii : string -> stringcapitalize_ascii s is s with the first character set to uppercase, using the US-ASCII character set.
val uncapitalize_ascii : string -> stringuncapitalize_ascii s is s with the first character set to lowercase, using the US-ASCII character set.
Traversing
val iter : f:(char -> unit) -> string -> unititer ~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 : f:(int -> char -> unit) -> string -> unititeri is like iter, but the function is also given the corresponding character index.
Searching
val index_from : string -> int -> char -> intindex_from s i c is the index of the first occurrence of c in s after position i.
val index_from_opt : string -> int -> char -> int optionindex_from_opt s i c is the index of the first occurrence of c in s after position i (if any).
val rindex_from : string -> int -> char -> intrindex_from s i c is the index of the last occurrence of c in s before position i+1.
val rindex_from_opt : string -> int -> char -> int optionrindex_from_opt s i c is the index of the last occurrence of c in s before position i+1 (if any).
val index : string -> char -> intval index_opt : string -> char -> int optionval rindex : string -> char -> intval rindex_opt : string -> char -> int optionStrings and Sequences
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.
val to_seqi : t -> (int * char) Seq.tto_seqi s is like to_seq but also tuples the corresponding index.
of_seq s is a string made of the sequence's characters.
UTF decoding and validations
UTF-8
get_utf_8_uchar b i decodes an UTF-8 character at index i in b.
val is_valid_utf_8 : t -> boolis_valid_utf_8 b is true if and only if b contains valid UTF-8 data.
UTF-16BE
get_utf_16be_uchar b i decodes an UTF-16BE character at index i in b.
val is_valid_utf_16be : t -> boolis_valid_utf_16be b is true if and only if b contains valid UTF-16BE data.
UTF-16LE
get_utf_16le_uchar b i decodes an UTF-16LE character at index i in b.
val is_valid_utf_16le : t -> boolis_valid_utf_16le b is true if and only if b contains valid UTF-16LE data.
val blit :
src:string ->
src_pos:int ->
dst:bytes ->
dst_pos:int ->
len:int ->
unitblit ~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.
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 -> intget_uint8 b i is b's unsigned 8-bit integer starting at character index i.
val get_int8 : string -> int -> intget_int8 b i is b's signed 8-bit integer starting at character index i.
val get_uint16_ne : string -> int -> intget_uint16_ne b i is b's native-endian unsigned 16-bit integer starting at character index i.
val get_uint16_be : string -> int -> intget_uint16_be b i is b's big-endian unsigned 16-bit integer starting at character index i.
val get_uint16_le : string -> int -> intget_uint16_le b i is b's little-endian unsigned 16-bit integer starting at character index i.
val get_int16_ne : string -> int -> intget_int16_ne b i is b's native-endian signed 16-bit integer starting at character index i.
val get_int16_be : string -> int -> intget_int16_be b i is b's big-endian signed 16-bit integer starting at character index i.
val get_int16_le : string -> int -> intget_int16_le b i is b's little-endian signed 16-bit integer starting at character index i.
val get_int32_ne : string -> int -> int32get_int32_ne b i is b's native-endian 32-bit integer starting at character index i.
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.
val seeded_hash : int -> t -> intval get_int32_be : string -> int -> int32get_int32_be b i is b's big-endian 32-bit integer starting at character index i.
val get_int32_le : string -> int -> int32get_int32_le b i is b's little-endian 32-bit integer starting at character index i.
val get_int64_ne : string -> int -> int64get_int64_ne b i is b's native-endian 64-bit integer starting at character index i.
val get_int64_be : string -> int -> int64get_int64_be b i is b's big-endian 64-bit integer starting at character index i.
val get_int64_le : string -> int -> int64get_int64_le b i is b's little-endian 64-bit integer starting at character index i.