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 i
th 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 i
th 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 -> string
make n c
is a string of length n
with each index holding the character c
.
val init : int -> f:(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).
val length : string -> int
length s
is the length (number of bytes/characters) of s
.
val get : string -> int -> char
get s i
is the character at index i
in s
. This is the same as writing s.[i]
.
val of_bytes : bytes -> string
Return a new string that contains the same bytes as the given byte sequence.
val to_bytes : string -> bytes
Return a new byte sequence that contains the same bytes as the given string.
val blit :
src:string ->
src_pos:int ->
dst:bytes ->
dst_pos:int ->
len:int ->
unit
Concatenating
Note. The Stdlib.(^)
binary operator concatenates two strings.
val concat : sep:string -> string list -> string
concat ~sep ss
concatenates the list of strings ss
, inserting the separator string sep
between each.
val cat : string -> string -> string
cat s1 s2
concatenates s1 and s2 (s1 ^ s2
).
Predicates and comparisons
val equal : t -> t -> bool
equal s0 s1
is true
if and only if s0
and s1
are character-wise equal.
val 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
starts_with
~prefix s
is true
if and only if s
starts with prefix
.
val ends_with : suffix:string -> string -> bool
ends_with
~suffix s
is true
if and only if s
ends with suffix
.
val contains_from : string -> int -> char -> bool
contains_from s start c
is true
if and only if c
appears in s
after position start
.
val 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
.
val contains : string -> char -> bool
val sub : string -> pos:int -> len: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
.
val split_on_char : sep:char -> string -> string list
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.
val map : f:(char -> char) -> string -> string
map 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 -> string
mapi ~f s
is like map
but the index of the character is also passed to f
.
val fold_left : f:('acc -> char -> 'acc) -> init:'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
.
val fold_right : f:(char -> 'acc -> 'acc) -> string -> init:'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
.
val for_all : f:(char -> bool) -> string -> bool
for_all p s
checks if all characters in s
satisfy the predicate p
.
val exists : f:(char -> bool) -> string -> bool
exists p s
checks if at least one character of s
satisfies the predicate p
.
val trim : string -> string
trim s
is s
without leading and trailing whitespace. Whitespace characters are: ' '
, '\x0C'
(form feed), '\n'
, '\r'
, and '\t'
.
val 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).
val uppercase_ascii : string -> string
uppercase_ascii s
is s
with all lowercase letters translated to uppercase, using the US-ASCII character set.
val lowercase_ascii : string -> string
lowercase_ascii s
is s
with all uppercase letters translated to lowercase, using the US-ASCII character set.
val capitalize_ascii : string -> string
capitalize_ascii s
is s
with the first character set to uppercase, using the US-ASCII character set.
val uncapitalize_ascii : string -> string
uncapitalize_ascii s
is s
with the first character set to lowercase, using the US-ASCII character set.
Traversing
val iter : f:(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 : f:(int -> char -> unit) -> string -> unit
iteri
is like iter
, but the function is also given the corresponding character index.
Searching
val 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
.
val index_from_opt : string -> int -> char -> int option
index_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 -> int
rindex_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 option
rindex_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 -> int
val index_opt : string -> char -> int option
val rindex : string -> char -> int
val rindex_opt : string -> char -> int option
Strings 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.t
to_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 -> bool
is_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 -> bool
is_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 -> 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.
val get_uint8 : string -> int -> int
get_uint8 b i
is b
's unsigned 8-bit integer starting at character index i
.
val get_int8 : string -> int -> int
get_int8 b i
is b
's signed 8-bit integer starting at character index i
.
val 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
.
val 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
.
val 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
.
val 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
.
val 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
.
val 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
.
val get_int32_ne : string -> int -> int32
get_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 -> int
val get_int32_be : string -> int -> int32
get_int32_be b i
is b
's big-endian 32-bit integer starting at character index i
.
val get_int32_le : string -> int -> int32
get_int32_le b i
is b
's little-endian 32-bit integer starting at character index i
.
val get_int64_ne : string -> int -> int64
get_int64_ne b i
is b
's native-endian 64-bit integer starting at character index i
.
val get_int64_be : string -> int -> int64
get_int64_be b i
is b
's big-endian 64-bit integer starting at character index i
.
val get_int64_le : string -> int -> int64
get_int64_le b i
is b
's little-endian 64-bit integer starting at character index i
.