StringLabels(3)

💖 NAME 💖

okay bestie, welcome to StringLabels(3) — the labeled-string module that's about to be your new fave. No cap, it's giving “strings but make it labeled” and we're gonna break it down together. ✨

🧚 Module 🧚

Module StringLabels
 : sig end

✨ Documentation ✨

Module StringLabels
 : sig end

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. (yes, bestie, bytes are getting called characters — it's a whole thing, just go with it 🫶)

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 🎀

• type t = string — The type for strings.
• 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 -> 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).
  • 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 : src:string -> src_pos:int -> dst:bytes -> dst_pos:int -> len:int -> unit — Same as Bytes.blit_string which should be preferred.

💞 Concatenating 💞

Note. The (^) 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.
  • 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 4.05
• 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 -> 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.
  • 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 StringLabels.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 StringLabels.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 StringLabels.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 StringLabels.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 : 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.
  • Since 4.05

🔥 Transforming 🔥

• 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.
  • Since 4.00
• val mapi : f:(int -> char -> char) -> string -> string — mapi ~f s is like StringLabels.map but the index of the character is also passed to f.
  • Since 4.02
• 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. Since 4.13.
• 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. Since 4.13.
• val for_all : f:(char -> bool) -> string -> bool — for_all p s checks if every single character in s satisfies the predicate p. Since 4.13.
• val exists : f:(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. The 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 4.05.
• val lowercase_ascii : string -> string — lowercase_ascii s is s with all uppercase letters translated to lowercase, using the US-ASCII character set. Since 4.05.
• val capitalize_ascii : string -> string — capitalize_ascii s is s with the first character set to uppercase, using the US-ASCII character set. Since 4.05.
• val uncapitalize_ascii : string -> string — uncapitalize_ascii s is s with the first character set to lowercase, using the US-ASCII character set. Since 4.05.

✨ 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 StringLabels.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 🔎

Bestie 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.

• 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 -> f:(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 -> f:(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 💅

Bestie 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 swaps the first occurrence of sub in s for by, 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 swaps the last occurrence of sub in s for by, 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 swaps every non-overlapping occurrence of sub in s for by, 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 StringLabels.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 StringLabels.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.

Okay so before we get into the actual functions, there's some lore you need to know 👀

Every single function below raises Invalid_argument if the characters it needs at index i to decode the integer just... aren't there. Like if you're asking for 8 bytes starting at index 3 of a 5-char string, she will not be nice about it. 😭

Little-endian (resp. big-endian) encoding means that least (resp. most) significant bytes are stored first — yes bestie, the byte order is literally the whole personality of the encoding. Big-endian is also known as network byte order (iykyk). Native-endian encoding is either little-endian or big-endian depending on Sys.big_endian — so it's giving "wherever you run it, that's the vibe" energy.

32-bit and 64-bit integers are represented by the int32 and int64 types, and they can be interpreted either as signed or unsigned numbers. Flexible icon behavior, honestly. ✨

8-bit and 16-bit integers are represented by the int type, which has more bits than the binary encoding. Those extra bits are sign-extended (or zero-extended) for functions which decode 8-bit or 16-bit integers and represented them with int values — think of it as padding out the outfit so it fits the bigger type 🎀

---

the actual functions, let's go 💅

• 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 👑

---

Also ngl — some of these decoders are literally int*32* and int*64* island and int*16* land, and yes, they're giving "why is int here doing overtime," but the manpage says what the manpage says. Go off, int. 💀

And that's a wrap on this page, bestie! 🎉 This was StringLabels(3), brought to you by OCamldoc, with a version date of 2026-06-22 — so print it out, put it on your wall, and go decode some bytes responsibly 🫶💖✨