String(3)

💖 String(3) — Strings 💖

OCaml library manual · String(3) · part 1, bestie ✨

🌸 NAME 🌸

String — Strings. That's the whole vibe: this is the module page for OCaml strings, and it is long, so buckle up 🎀

🧩 Module 🧩

Module String : sig end

📚 Documentation 📚

Okay so this is the String module, and here's the tea: 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. Yes, bytes are getting called characters, we don't make the rules 💅

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. Periodt, they stay put 🔒

The labeled version of this module can be used as described in the StdLabels module.

🧵 Strings 🧵

• type t = string — the type for strings. That's it, that's the type 💖
• val 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 — 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. Raises Invalid_argument if n < 0 or n > Sys.max_string_length .
• val empty : string — the empty string. Since 4.13 ✨
• 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] . Raises Invalid_argument if i not an index of s .
• val of_char : char -> string — of_char c is c as a string. Since 5.5 🎀
• val 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 — 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 — Same as Bytes.blit_string which should be preferred. Lowkey just use the Bytes one, bestie 💅

🎀 Concatenating 🎀

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

• val concat : string -> string list -> 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 — cat s1 s2 concatenates s1 and s2 ( s1 ^ s2 ). Since 4.13. Raises Invalid_argument if the result is longer than Sys.max_string_length bytes.

🔍 Predicates and comparisons 🔍

• val 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 — compare s0 s1 sorts s0 and s1 in lexicographical order. compare behaves like compare on strings but may be more efficient.
• val is_empty : string -> bool — is_empty s is true if and only if s is an empty string. Since 5.5 🫶
• val 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 — ends_with ~suffix s is true if and only if s ends with suffix . Since 4.13 🌟
• val includes : affix:string -> string -> bool — includes affix s is true if and only if affix occurs in s . Since 5.5. Note. To test the same affix string multiple times, partially applying the ~affix argument and using the resulting function repeatedly is more efficient. iykyk 🧚
• 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 . Raises Invalid_argument if start is not a valid position in s .
• 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 . Raises Invalid_argument if stop < 0 or stop+1 is not a valid position in s .
• val contains : string -> char -> bool — contains s c is String.contains_from s 0 c .

✂️ Extracting substrings ✂️

• val 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 .

🪄 Splitting strings 🪄

🌈 Splitting with magnitudes 🌈

• val 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 💖
• val 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 💖
• val 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 🌸
• val 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 🌸
• val cut_first : int -> string -> string * string — cut_first n v is (take_first n v, drop_first n v) . Since 5.5 🎀
• val 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 🦋

• val 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 ✨
• val 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 ✨
• val 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 🌟
• val 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 🌟
• val 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 💜
• val 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.

• val 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 String.find_first . If sep is "" , this is Some ("", s) . The invariant concat sep [left; right] = s holds. Since 5.5 🍓
• val 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 String.find_last . If sep is "" , this is Some (s, "") . The invariant concat sep [left; right] = s holds. Since 5.5 🍓
• val 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 String.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 🌈
• val 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 String.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 🌈
• val 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 🦋

• val 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 — mapi f s is like String.map but the index of the character is also passed to f . Since 4.02 🎀
• val 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 — 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 — for_all p s checks if all characters in s satisfy the predicate p.

✨ Since 4.13

• val 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 — trim s is s without leading and trailing whitespace, bestie. And "whitespace" here literally means: ' ', '\x0C' (form feed), '\n', '\r', and '\t'. That's the whole list, no extras, periodt.

✨ Since 4.00

• val escaped : string -> string — escaped s is s but with special characters represented by escape sequences, following the lexical conventions of OCaml. Iconic behavior honestly.

Every character outside the US-ASCII printable range [0x20;0x7E] gets escaped, plus 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). Cute lil inverse relationship 💖

• Raises Invalid_argument if the result is longer than Sys.max_string_length bytes. 😭

• val 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 — 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 — 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 — 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 — 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]; (). Like walking the whole string, girly pop, one char at a time.
• val iteri : (int -> char -> unit) -> string -> unit — iteri is like String.iter, but the function is also given the corresponding character index. Two-for-one deal, we love to see it 🫶

✨ Since 4.00

🔎 Finding indices

• val 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. So don't come for her when she returns None, that's correct 💅

✨ Since 5.5

• Raises Invalid_argument if start is not a valid position of s.

• val 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). Backwards search era 🌸

✨ Since 5.5

• Raises Invalid_argument if start is not a valid position of s.

• 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.
  • 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 — index_from_opt s i c is the index of the first occurrence of c in s after position i (if any). The _opt version so you don't get yeeted by an exception, we stan 😌

✨ Since 4.05

• Raises Invalid_argument if i is not a valid position in s.

• 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. Note the i+1, that's not a typo, keep it exactly like that bestie.
  • 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 — 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

• Raises Invalid_argument if i+1 is not a valid position in s.

• val index : string -> char -> int — index s c is String.index_from s 0 c. That's the whole vibe check.
• val index_opt : string -> char -> int option — index_opt s c is String.index_from_opt s 0 c.

✨ Since 4.05

• val rindex : string -> char -> int — rindex s c is String.rindex_from s (length s - 1) c. Starting from the end, main character energy 👑
• val 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. Little performance glow-up, no cap 💅

• val 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. Read that twice, it's lowkey doing a lot 🫶

✨ Since 5.5

• Raises Invalid_argument if start is not a valid position of s.

• val 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

• Raises Invalid_argument if start is not a valid position of s.

• val 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

• Raises Invalid_argument if start is not a valid position of s.

• val 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

• Raises Invalid_argument if start is not a valid position of s.

🔁 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.

• val 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. Sneaky little insert, love her 🙈

✨ Since 5.5

• Raises Invalid_argument if start is not a valid position of s.

• val 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

• Raises Invalid_argument if start is not a valid position of s.

• val 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

• Raises Invalid_argument if start is not a valid position of s.

🦋 Strings and Sequences

• val to_seq : t -> char Seq.t — 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 — to_seqi s is like String.to_seq but also tuples the corresponding index. Extra info, zero extra effort 💅

✨ Since 4.07

• val 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

• val 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 — 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 — 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

• val 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 — 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 — 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. Speed queen behavior ⚡

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. Do not come for her about that, it's literally documented 💀

✨ Since 5.4

• val 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 String.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. Yes, we're going from vibes to raw bytes, buckle up 🎀

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 — yes bestie, that's the same thing you see in all your networking code, it's not a different vibe 💅. Native-endian encoding is either little-endian or big-endian depending on Sys.big_endian, so it's like, whatever your machine woke up feeling today.

---

The int types, explained

32-bit and 64-bit integers are represented by the int32 and int64 types, which can be interpreted either as signed or unsigned numbers. Choose your fighter ✨

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. So like, the int is wearing a little extra padding, it's giving oversized hoodie energy 🫶

---

🌸 The getters, one by one 🌸

• val 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 — 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 — 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 — 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 — 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 — 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 — 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 — 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 — 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 — 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 — 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 — 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 — 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 — 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 — 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 — get_int64_le b i is b's little-endian 64-bit integer starting at character index i. Since 4.13

Quick vibe check, bestie: notice there's no get_uint32 or get_uint64 in this list — the 32-bit and 64-bit getters are the plain int32 / int64 ones up there, and the signed/unsigned call is on you. Don't go inventing functions that aren't on this page 🙈

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And that's the whole module, no cap — you read every single getter, both hash functions, and the endianness lore. Go decode something iconic ✨👑

This page came from the OCamldoc source, version-and-dated 2026-06-22, and it's the String(3) manual page 💖 slay, go touch some grass now, you've earned it 🌸🦋