Stdlib.String(3)

okay bestie, buckle up: this is Stdlib.String(3) from the OCaml library, aka the main-character string module. no cap, every signature, caveat, and tiny detail is staying exactly where it belongs 💅✨

🌸 NAME 🌸

Stdlib.String — literally “no description” in the original, and I’m not inventing one 💀

🧩 Module 🧩

Module Stdlib.String

📚 Documentation 📚

Module String : (module Stdlib__String)

💖 Strings 💖

• type t = string — the type for strings. iconic. ✨
• 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. 🫶

✨ 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. 💡 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
• 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 ✂️

okay so this is the splitting era, and yes every flavor is here 💅

📏 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. Whitespace characters are: ' ', '\x0C' (form feed), '\n', '\r', and '\t'. 🎀 Since 4.00
• 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 (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). 🚨 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) — uncapitalize_ascii is literally the lowercase-first-letter glow-up for a string 💅

• val uncapitalize_ascii : string -> string — uncapitalize_ascii s is s with the first character set to lowercase, using the US-ASCII character set. No cap, it's just that first char catching lowercase energy. ✨

🌀 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]; ()

• val iteri : (int -> char -> unit) -> string -> unit — iteri is like String.iter, but the function is also given the corresponding character index. 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. 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). 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). 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.

⚠️ 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. 🎯
• 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. 🌸
• 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. iykyk — less repeated setup, more slay.

• 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. 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. Same energy, less repetitive setup.

• 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. 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. 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. 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

• 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.

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. 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

val get_int16_le : string -> int -> int16

get_int16_le b i pulls the little-endian signed 16-bit integer sitting at character index i in b. 💅 Little-endian means the smallest byte comes first — she's quirky like that.

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. Native-endian = whatever your machine woke up feeling today, bestie 🫶 so don't go assuming, iykyk.

Since 4.13

val hash : t -> int

An unseeded hash function for strings, and the tea is it gives the exact same output value as Hashtbl.hash. ✨ The whole point? It lets this module get passed as an argument to the functor Hashtbl.Make. Matching energy, no cap.

Since 5.0

val seeded_hash : int -> t -> int

A seeded hash function for strings, output value identical to Hashtbl.seeded_hash. 💖 Same deal — this is what lets this module be passed as an argument to the functor Hashtbl.MakeSeeded. Seeded besties stay winning.

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. Big-endian: biggest byte first, main character energy 👑.

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. Tiny byte goes first, we stan a humble queen 🎀.

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. Bigger number, same chaotic energy — it just matches your platform's vibe 🌈.

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. Sixty-four whole bits, big end first, absolutely serving 🍓.

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. Last but never least — little-end first, forever iconic 💜.

Since 4.13

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And that's a wrap on the whole page, bestie!! 🦋 This lil masterpiece was brought to you by OCamldoc, dated 2026-06-22, living at Stdlib.String(3). Go forth and read them bytes responsibly 💖✨