package uuseg

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Unicode text segmentation.

Uuseg segments Unicode text. It implements the locale independent Unicode text segmentation algorithms to detect grapheme cluster, word and sentence boundaries and the Unicode line breaking algorithm to detect line break opportunities.

The module is independent from any IO mechanism or Unicode text data structure and it can process text without a complete in-memory representation.

The supported Unicode version is determined by the unicode_version value.

Consult the basics, limitations and examples of use.

Release 0.9.0 – Daniel Bünzli <daniel.buenzl>


Unicode characters

type uchar = int

The type for Unicode characters. A value of this type must be an Unicode scalar value which is an integer value in the ranges 0x0000...0xD7FF and 0xE000...0x10FFFF.

val is_uchar : int -> bool

is_uchar cp is true iff cp is an Unicode scalar value.

val unicode_version : string

unicode_version is the Unicode version supported by Uuseg.


type custom

The type for custom segmenters. See custom.

type boundary = [
  1. | `Grapheme_cluster
  2. | `Word
  3. | `Sentence
  4. | `Line_break
  5. | `Custom of custom

The type for boundaries.

  • `Grapheme_cluster determines extended grapheme clusters boundaries according to UAX 29 (corresponds, for most scripts, to user-perceived characters).
  • `Word determines word boundaries according to UAX 29.
  • `Sentence determines sentence boundaries according to UAX 29.
  • `Line_break determines mandatory line breaks and line break opportunities according to UAX 14.
val pp_boundary : Format.formatter -> boundary -> unit

pp_boundary ppf b prints an unspecified representation of b on ppf.

type t

The type for Unicode text segmenters.

type ret = [
  1. | `Boundary
  2. | `Uchar of uchar
  3. | `Await
  4. | `End

The type for segmenter results. See add.

val create : [< boundary ] -> t

create b is an Unicode text segmenter for boundaries of type b.

val boundary : t -> boundary

boundary s is the type of boundaries detected by s.

val add : t -> [ `Uchar of uchar | `Await | `End ] -> ret

add s v is:

  • `Boundary if there is a boundary at that point in the sequence of characters. The client must then call add with `Await until `Await is returned.
  • `Uchar u if u is the next character in the sequence. The client must then call add with `Await until `Await is returned.
  • `Await when the segmenter is ready to add a new `Uchar or `End.
  • `End when `End was added and all `Boundary and `Uchar were output.

For v use `Uchar u to add a new character to the sequence to segment and `End to signal the end of sequence. After adding one of these two values always call add with `Await until `Await or `End is returned.

  • raises Invalid_argument

    if `Uchar or `End is added while that last add did not return `Await or if an `Uchar or `End is added after an `End was already added.

    Warning. add deals with Unicode scalar values. If you are handling foreign data you must assert that before with is_uchar.

val mandatory : t -> bool

mandatory s is true if the last `Boundary returned by add was mandatory. This function only makes sense for `Line_break segmenters or `Custom segmenters that sport that notion. For other segmenters or if no `Boundary was returned so far, true is returned.

val copy : t -> t

copy s is a copy of s in its current state. Subsequent adds on s do not affect the copy.

val pp_ret : Format.formatter -> [< ret ] -> unit

pp_ret ppf v prints an unspecified representation of v on ppf.

Custom segmenters

val custom : ?mandatory:('a -> bool) -> name:string -> create:(unit -> 'a) -> copy:('a -> 'a) -> add:('a -> [ `Uchar of uchar | `Await | `End ] -> ret) -> unit -> custom

create ~mandatory ~name ~create ~copy ~add is a custom segmenter.

  • name is a name to identify the segmenter.
  • create is called when the segmenter is created it should return a custom segmenter value.
  • copy is called with the segmenter value whenever the segmenter is copied. It should return a copy of the segmenter value.
  • mandatory is called with the segmenter value to define the result of the mandatory function. Defaults always returns true.
  • add is called with the segmenter value to define the result of the add value. The returned value should respect the semantics of add. Use the functions err_exp_await and err_ended to raise Invalid_argument exception in adds error cases.
val err_exp_await : [< ret ] -> 'a

err_exp_await fnd should be used by custom segmenters when the client tries to add an `Uchar or `End while the last returned value was not an `Await.

val err_ended : [< ret ] -> 'a

err_ended () should be used by custom segmenter when the client tries to add `Uchar or `End after `End was already added.


A `Grapheme_cluster segmenter will always consume only a small bounded amount of memory on any text. Other segmenters will also do so on non-degenerate text, but it's possible to feed them with input that will make them buffer an arbitrary amount of characters.


A segmenter is a stateful filter that inputs a sequence of characters and outputs the same sequence except characters are interleaved with `Boundary values whenever the segmenter detects a boundary.

The function create returns a new segmenter for a given boundary type:

let words = Uuseg.create `Word

To add characters to the sequence to segment, call add on words with `Uchar _. To end the sequence call add on words with `End. The segmented sequence of characters is returned character by character, interleaved with `Boundary values at the appropriate places, by the successive calls to add.

The client and the segmenter must wait on each other to limit internal buffering: each time the client adds to the sequence by calling add with `Uchar or `End it must continue to call add with `Await until the segmenter returns `Await or `End. In practice this leads to the following kind of control flow:

let rec add acc v = match Uuseg.add words v with
| `Uchar u -> add (`Uchar u :: acc) `Await
| `Boundary -> add (`B :: acc) `Await
| `Await | `End -> acc

For example to segment the sequence <U+0041, U+0020, U+0042> ("a b") to a list of characters interleaved with `B values on word boundaries we can write:

let seq = [`Uchar 0x0041; `Uchar 0x0020; `Uchar 0x0042]
let seq_words = List.rev (add (List.fold_left add [] seq) `End)


utf_8_segments seg s is the list of UTF-8 encoded seg segments of the UTF-8 encoded string s. This example uses Uutf to fold over the characters of s and to encode the characters in a standard OCaml buffer .

let utf_8_segments seg s =
  let b = Buffer.create 42 in
  let flush_segment acc =
    let segment = Buffer.contents b in
    Buffer.clear b; if segment = "" then acc else segment :: acc
  let seg = Uuseg.create (seg :> Uuseg.boundary) in
  let rec add acc v = match Uuseg.add seg v with
  | `Uchar u -> Uutf.Buffer.add_utf_8 b u; add acc `Await
  | `Boundary -> add (flush_segment acc) `Await
  | `Await -> acc
  let rec uchar acc _ = function
  | `Uchar _ as u -> add acc u
  | `Malformed _ -> add acc (`Uchar Uutf.u_rep)
  List.rev (flush_segment (add (Uutf.String.fold_utf_8 uchar [] s) `End))

Note that this function can derived directly from Uuseg_string.fold_utf_8.


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