Module Uuseg

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 v10.0.0 – %%MAINTAINER%%



val unicode_version : string

unicode_version is the Unicode version supported by Uuseg.

type custom

The type for custom segmenters. See custom.

type boundary = [
| `Grapheme_cluster
| `Word
| `Sentence
| `Line_break
| `Custom of custom

The type for boundaries.

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 = [
| `Boundary
| `Uchar of Uchar.t
| `Await
| `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.t | `Await | `End ] ‑> ret

add s v is:

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.

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.t | `Await | `End ] ‑> ret) ‑> unit ‑> custom

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

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 uchar = `Uchar (Uchar.of_int u)
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. Note that this function can be derived directly from Uuseg_string.fold_utf_8.

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))