String(3) OCaml library String(3)

# [NAME](#NAME)

String - Strings.

# [Module](#Module)

Module String

# [Documentation](#Documentation)

Module **String**  
: **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.

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. OCaml strings used to be modifiable in place, for instance via the **String.set** and **String.blit** functions. This use is nowadays only possible when the compiler is put in "unsafe-string" mode by giving the **-unsafe-string** command-line option. This compatibility mode makes the types **string** and **bytes** (see **Bytes.t** ) interchangeable so that functions expecting byte sequences can also accept strings as arguments and modify them.

The distinction between **bytes** and **string** was introduced in OCaml 4.02, and the "unsafe-string" compatibility mode was the default until OCaml 4.05. Starting with 4.06, the compatibility mode is opt-in; we intend to remove the option in the future.

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

## [Strings](#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 -> (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** 4.02.0

**Raises Invalid\_argument** if **n < 0** or **n >** **Sys.max\_string\_length** .

*val empty* : **string**

The empty string.

**Since** 4.13.0

*val of\_bytes* : **bytes -> string**

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

**Since** 4.13.0

*val to\_bytes* : **string -> bytes**

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

**Since** 4.13.0

*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** .

## [Concatenating](#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** 4.13.0

**Raises Invalid\_argument** if the result is longer than **Sys.max\_string\_length** bytes.

## [Predicates and comparisons](#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** 4.03.0 (4.05.0 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 starts\_with* : **prefix:string -> string -> bool**

**starts\_with** **~prefix s** is **true** if and only if **s** starts with **prefix** .

**Since** 4.13.0

*val ends\_with* : **suffix:string -> string -> bool**

**ends\_with** **~suffix s** is **true** if and only if **s** ends with **suffix** .

**Since** 4.13.0

*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](#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** .

*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** .

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** 4.04.0 (4.05.0 in StringLabels)

## [Transforming](#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** 4.00.0

*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** 4.02.0

*val fold\_left* : **('a -> char -> 'a) -> 'a -> string -> 'a**

**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** 4.13.0

*val fold\_right* : **(char -> 'a -> 'a) -> string -> 'a -> 'a**

**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** 4.13.0

*val for\_all* : **(char -> bool) -> string -> bool**

**for\_all p s** checks if all characters in **s** satisfy the predicate **p** .

**Since** 4.13.0

*val exists* : **(char -> bool) -> string -> bool**

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

**Since** 4.13.0

*val trim* : **string -> string**

**trim s** is **s** without leading and trailing whitespace. Whitespace characters are: **' '** , **'\\x0C'** (form feed), **'\\n'** , **'\\r'** , and **'\\t'** .

**Since** 4.00.0

*val escaped* : **string -> string**

**escaped s** is **s** with special characters represented by escape sequences, following the lexical conventions of OCaml.

All characters outside the US-ASCII printable range \[0x20;0x7E] are escaped, as well as backslash (0x2F) and double-quote (0x22).

The function **Scanf.unescaped** is a left inverse of **escaped** , i.e. **Scanf.unescaped (escaped s) = s** for any string **s** (unless **escaped s** fails).

**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** 4.03.0 (4.05.0 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** 4.03.0 (4.05.0 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** 4.03.0 (4.05.0 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** 4.03.0 (4.05.0 in StringLabels)

## [Traversing](#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** 4.00.0

## [Searching](#Searching)

*val index\_from* : **string -> int -> char -> int**

**index\_from s i c** is the index of the first occurrence of **c** in **s** after position **i** .

**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

## [Strings and Sequences](#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. In **"unsafe-string"** mode, modifications of the string during iteration will be reflected in the sequence.

**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_decoding_and_validations)

## [UTF-8](#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](#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](#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.

## [Deprecated functions](#Deprecated_functions)

*val create* : **int -> bytes**

**Deprecated.** This is a deprecated alias of **Bytes.create** / **BytesLabels.create** .

**create n** returns a fresh byte sequence of length **n** . The sequence is uninitialized and contains arbitrary bytes.

**Raises Invalid\_argument** if **n < 0** or **n >** **Sys.max\_string\_length** .

*val set* : **bytes -> int -> char -> unit**

**Deprecated.** This is a deprecated alias of **Bytes.set** / **BytesLabels.set** .

**set s n c** modifies byte sequence **s** in place, replacing the byte at index **n** with **c** . You can also write **s.\[n] <- c** instead of **set s n c** .

**Raises Invalid\_argument** if **n** is not a valid index in **s** .

*val blit* : **string -> int -> bytes -> int -> int -> unit**

**blit src src\_pos dst dst\_pos len** copies **len** bytes from the string **src** , starting at index **src\_pos** , to byte sequence **dst** , starting at character number **dst\_pos** .

**Raises Invalid\_argument** if **src\_pos** and **len** do not designate a valid range of **src** , or if **dst\_pos** and **len** do not designate a valid range of **dst** .

*val copy* : **string -> string**

**Deprecated.** Because strings are immutable, it doesn't make much sense to make identical copies of them.

Return a copy of the given string.

*val fill* : **bytes -> int -> int -> char -> unit**

**Deprecated.** This is a deprecated alias of **Bytes.fill** / **BytesLabels.fill** .

**fill s pos len c** modifies byte sequence **s** in place, replacing **len** bytes by **c** , starting at **pos** .

**Raises Invalid\_argument** if **pos** and **len** do not designate a valid substring of **s** .

*val uppercase* : **string -> string**

**Deprecated.** Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with all lowercase letters translated to uppercase, including accented letters of the ISO Latin-1 (8859-1) character set.

*val lowercase* : **string -> string**

**Deprecated.** Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with all uppercase letters translated to lowercase, including accented letters of the ISO Latin-1 (8859-1) character set.

*val capitalize* : **string -> string**

**Deprecated.** Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with the first character set to uppercase, using the ISO Latin-1 (8859-1) character set..

*val uncapitalize* : **string -> string**

**Deprecated.** Functions operating on Latin-1 character set are deprecated.

Return a copy of the argument, with the first character set to lowercase, using the ISO Latin-1 (8859-1) character set.

## [Binary decoding of integers](#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** 4.13.0

*val get\_int8* : **string -> int -> int**

**get\_int8 b i** is **b** 's signed 8-bit integer starting at character index **i** .

**Since** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

*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** 4.13.0

2026-10-27 OCamldoc

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Document: String(3o) from ocaml-4 4.14.4-r4 (Chainguard OS)
Source: https://github.com/ocaml/ocaml