Module Stdlib.StringSource

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

Sourcetype t = string

The type for strings.

Sourceval make : int -> char -> string

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

Sourceval 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
Sourceval empty : string

The empty string.

  • since 4.13
Sourceval length : string -> int

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

Sourceval get : string -> int -> char

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

Sourceval of_char : char -> string

of_char c is c as a string.

  • since 5.5
Sourceval of_bytes : bytes -> string

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

  • since 4.13
Sourceval to_bytes : string -> bytes

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

  • since 4.13
Sourceval 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.

Sourceval concat : string -> string list -> string

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

Sourceval cat : string -> string -> string

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

  • since 4.13

Predicates and comparisons

Sourceval 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)
Sourceval 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.

Sourceval is_empty : string -> bool

is_empty s is true if and only if s is an empty string.

  • since 5.5
Sourceval starts_with : prefix:string -> string -> bool

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

  • since 4.13
Sourceval ends_with : suffix:string -> string -> bool

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

  • since 4.13
Sourceval includes : affix:string -> string -> bool

includes affix s is true if and only if affix occurs in s.

Note. To test the same affix string multiple times, partially applying the ~affix argument and using the resulting function repeatedly is more efficient.

  • since 5.5
Sourceval contains_from : string -> int -> char -> bool

contains_from s start c is true if and only if c appears in s after position start.

Sourceval 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.

Sourceval contains : string -> char -> bool

contains s c is String.contains_from s 0 c.

Extracting substrings

Sourceval 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.

Splitting strings

Splitting with magnitudes

Sourceval take_first : int -> string -> string

take_first n s are the first n bytes of s. This is s if n >= length s and "" if n <= 0.

  • since 5.5
Sourceval take_last : int -> string -> string

take_last n s are the last n bytes of s. This is s if n >= length s and "" if n <= 0.

  • since 5.5
Sourceval drop_first : int -> string -> string

drop_first n s is s without the first n bytes of s. This is "" if n >= length s and s if n <= 0.

  • since 5.5
Sourceval drop_last : int -> string -> string

drop_last n s is s without the last n bytes of s. This is "" if n >= length s and s if n <= 0.

  • since 5.5
Sourceval cut_first : int -> string -> string * string

cut_first n v is (take_first n v, drop_first n v).

  • since 5.5
Sourceval cut_last : int -> string -> string * string

cut_last n v is (drop_last n v, take_last n v).

  • since 5.5

Splitting with predicates

Sourceval take_first_while : (char -> bool) -> string -> string

take_first_while p s is the first consecutive bytes of s satisfying the predicate p.

  • since 5.5
Sourceval take_last_while : (char -> bool) -> string -> string

take_last_while p s is the last consecutive bytes of s satisfying the predicate p.

  • since 5.5
Sourceval drop_first_while : (char -> bool) -> string -> string

drop_first_while p s is s without the first consecutive bytes of s satisfying the predicate p.

  • since 5.5
Sourceval drop_last_while : (char -> bool) -> string -> string

drop_last_while p s is s without the last consecutive bytes of s satisfying the predicate p.

  • since 5.5
Sourceval cut_first_while : (char -> bool) -> string -> string * string

cut_first_while p s is (take_first_while p s, drop_first_while p s).

  • since 5.5
Sourceval cut_last_while : (char -> bool) -> string -> string * string

cut_last_while p s is (drop_last_while p s, take_last_while p s).

  • since 5.5

Splitting with separators

Note. To split the same sep string multiple times, partially applying the ~sep argument of these functions and using the resulting function repeatedly is more efficient.

Sourceval split_first : sep:string -> string -> (string * string) option

split_first sep s is the pair Some (left, right) made of the two (possibly empty) substrings of s that are delimited by the first match of the separator sep in s or None if sep can't be found. Search for sep starts at position 0 and uses find_first.

If sep is "", this is Some ("", s).

The invariant concat sep [left; right] = s holds.

  • since 5.5
Sourceval split_last : sep:string -> string -> (string * string) option

split_last sep s is the pair Some (left, right) made of the two (possibly empty) substrings of s that are delimited by the last match of the separator sep in s or None if sep can't be found. Search for sep starts at position length s and uses find_last.

If sep is "", this is Some (s, "").

The invariant concat sep [left; right] = s holds.

  • since 5.5
Sourceval split_all : sep:string -> ?drop:(string -> bool) -> string -> string list

split_all sep s is the list of all substrings of s that are delimited by non-overlapping matches of the separator sep or the list [s] if sep can't be found. Search for sep starts at position 0 in increasing indexing order and uses find_all.

Substrings sub for which drop sub is true are not included in the result. drop defaults to Fun.const false.

If sep is "", this is [""; c0; ...; cn; ""] with ci the string of_char s.[i].

The invariant concat sep (split_all sep s) = s holds.

  • since 5.5
Sourceval rsplit_all : sep:string -> ?drop:(string -> bool) -> string -> string list

rsplit_all sep s is the list of all substrings of s that are delimited by non-overlapping matches of the separator sep or [s] if sep can't be found. Search for sep starts at position length s in decreasing indexing order and uses rfind_all.

Substrings sub for which drop sub is true are not included in the result. drop defaults to Fun.const false.

If sep is "", this is [""; c0; ...; cn; ""] with ci the string of_char s.[i].

The invariant concat sep (rsplit_all sep s) = s holds.

  • since 5.5
Sourceval split_on_char : 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.
  • since 4.04 (4.05 in StringLabels)

Transforming

Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval for_all : (char -> bool) -> string -> bool

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

  • since 4.13
Sourceval exists : (char -> bool) -> string -> bool

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

  • since 4.13
Sourceval 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
Sourceval 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 (0x5C) 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).

Sourceval 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)
Sourceval 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)
Sourceval 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)
Sourceval 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

Sourceval 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]; ().

Sourceval iteri : (int -> char -> unit) -> string -> unit

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

  • since 4.00

Finding indices

Sourceval find_first_index : (char -> bool) -> ?start:int -> string -> int option

find_first_index p start s is the index of the first character of s that satisfies predicate p at or after the index or position start (defaults to 0).

If start is length s, the result is always None.

  • since 5.5
Sourceval find_last_index : (char -> bool) -> ?start:int -> string -> int option

find_last_index p start s is the index of the last character of s that satisfies predicate p at or before the index or position start (defaults to length s).

  • since 5.5
Sourceval 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.

Sourceval 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).

  • since 4.05
Sourceval 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.

Sourceval 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).

  • since 4.05
Sourceval index : string -> char -> int

index s c is String.index_from s 0 c.

Sourceval index_opt : string -> char -> int option

index_opt s c is String.index_from_opt s 0 c.

  • since 4.05
Sourceval rindex : string -> char -> int

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

Sourceval rindex_opt : string -> char -> int option

rindex_opt s c is String.rindex_from_opt s (length s - 1) c.

  • since 4.05

Finding substrings

Note. To find the same sub string multiple times, partially applying the ~sub argument of these functions and using the resulting function repeatedly is more efficient

Sourceval find_first : sub:string -> ?start:int -> string -> int option

find_first sub start s is the starting position of the first occurrence of sub in s at or after the index or position start (defaults to 0).

If sub is "" the result is Some start. The result of the function is always a valid index of s except when sub is "" and start is length s.

  • since 5.5
Sourceval find_last : sub:string -> ?start:int -> string -> int option

find_last sub start s is the starting position of the last occurrence of sub in s at or before the index or position start (defaults to String.length s).

If sub is "" the result is Some start. The result of the function is always a valid index of s except when sub is "" and start is length s.

  • since 5.5
Sourceval find_all : sub:string -> (int -> 'acc -> 'acc) -> ?start:int -> string -> 'acc -> 'acc

find_all sub f start s acc, starting with acc, folds f by increasing index order over all non-overlapping starting positions of sub in s at or after the index or position start (defaults to 0). The result is acc if sub could not be found in s.

If sub is "", f gets invoked on all positions of s at or after start.

  • since 5.5
Sourceval rfind_all : sub:string -> (int -> 'acc -> 'acc) -> ?start:int -> string -> 'acc -> 'acc

rfind_all sub f start s acc, starting with acc, folds f by decreasing index order over all non-overlapping starting positions of sub in s at or before the index or position start (defaults to String.length s). The result is acc if sub could not be found in s.

If sub is "", f gets invoked on all positions of s at or before start.

  • since 5.5

Replacing substrings

Note. To replace the same sub string multiple times, partially applying the ~sub argument of these functions and using the resulting function repeatedly is more efficient.

Sourceval replace_first : sub:string -> by:string -> ?start:int -> string -> string

replace_first sub by start s replaces by by the first occurrence of sub in s at or after the index or position start (defaults to 0).

If sub is "", this inserts by at position start.

  • since 5.5
Sourceval replace_last : sub:string -> by:string -> ?start:int -> string -> string

replace_last sub by start s replaces by by the last occurrence of sub in s at or after the index or position start (defaults to String.length s).

If sub is "", this inserts by at position start.

  • since 5.5
Sourceval replace_all : sub:string -> by:string -> ?start:int -> string -> string

replace_all sub by start s replaces by by all non-overlapping occurrences of sub in s at or after the index or position start (defaults to 0). Occurrences are found in increasing indexing order.

If sub is "", this inserts by on all positions from start on.

  • since 5.5

Strings and Sequences

Sourceval to_seq : t -> char Seq.t

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

  • since 4.07
Sourceval to_seqi : t -> (int * char) Seq.t

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

  • since 4.07
Sourceval of_seq : char Seq.t -> t

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

  • since 4.07

UTF decoding and validations

UTF-8

Sourceval 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.

Sourceval 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

Sourceval 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.

Sourceval 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

Sourceval 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.

Sourceval is_valid_utf_16le : t -> bool

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

Spellchecking

Sourceval 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 Alignment (OSA) distance, not the Damerau-Levenshtein distance. With this function "ca" and "abc" have a distance of 3 not 2.

  • since 5.4
Sourceval spellcheck : ?max_dist:(string -> int) -> ((string -> unit) -> unit) -> string -> string list

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.

Sourceval get_uint8 : string -> int -> int

get_uint8 b i is b's unsigned 8-bit integer starting at character index i.

  • since 4.13
Sourceval get_int8 : string -> int -> int

get_int8 b i is b's signed 8-bit integer starting at character index i.

  • since 4.13
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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
Sourceval 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