package sedlex

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An OCaml lexer generator for Unicode


Dune Dependency






sedlex is a lexer generator for OCaml. It is similar to ocamllex, but supports Unicode. Unlike ocamllex, sedlex allows lexer specifications within regular OCaml source files. Lexing specific constructs are provided via a ppx syntax extension.

Published: 05 Dec 2021



Unicode-friendly lexer generator for OCaml.

This package is licensed by LexiFi under the terms of the MIT license.

sedlex was originally written by Alain Frisch and is now maintained as part of the ocaml-community repositories on github.


The API is documented here.


sedlex is a lexer generator for OCaml, similar to ocamllex, but supporting Unicode. Contrary to ocamllex, lexer specifications for sedlex are embedded in regular OCaml source files.

The lexers work with a new kind of "lexbuf", similar to ocamllex Lexing lexbufs, but designed to support Unicode, and abstracting from a specific encoding. A single lexer can work with arbitrary encodings of the input stream.

sedlex is the successor of the ulex project. Contrary to ulex which was implemented as a Camlp4 syntax extension, sedlex is based on the new "-ppx" technology of OCaml, which allow rewriting OCaml parse trees through external rewriters. (And what a better name than "sed" for a rewriter?)

As any -ppx rewriter, sedlex does not touch the concrete syntax of the language: lexer specifications are written in source file which comply with the standard grammar of OCaml programs. sedlex reuse the syntax for pattern matching in order to describe lexers (regular expressions are encoded within OCaml patterns). A nice consequence is that your editor (vi, emacs, ...) won't get confused (indentation, coloring) and you don't need to learn new priority rules. Moreover, sedlex is compatible with any front-end parsing technology: it works fine even if you use camlp4 or camlp5, with the standard or revised syntax.

Lexer specifications

sedlex adds a new kind of expression to OCaml: lexer definitions. The syntax for the new construction is:

  match%sedlex lexbuf with
  | R1 -> e1
  | Rn -> en
  | _  -> def


  [%sedlex match lexbuf with 
  | R1 -> e1
  | Rn -> en
  | _  -> def

(The first vertical bar is optional as in any OCaml pattern matching. Guard expressions are not allowed.)


  • lexbuf is an arbitrary lowercase identifier, which must refer to an existing value of type Sedlexing.lexbuf.

  • the Ri are regular expressions (see below);

  • the ei and def are OCaml expressions (called actions) of the same type (the type for the whole lexer definition).

Unlike ocamllex, lexers work on stream of Unicode codepoints, not bytes.

The actions can call functions from the Sedlexing module to extract (parts of) the matched lexeme, in the desired encoding.

Regular expressions are syntactically OCaml patterns:

  • "...." (string constant): recognize the specified string

  • '....' (character constant) : recognize the specified character

  • i (integer constant) : recognize the specified codepoint

  • '...' .. '...': character range

  • i1 .. i2: range between two codepoints

  • R1 | R2 : alternation

  • R, R2, ..., Rn : concatenation

  • Star R : Kleene star (0 or more repetition)

  • Plus R : equivalent to R, R*

  • Opt R : equivalent to ("" | R)

  • Rep (R, n) : equivalent to R{n}

  • Rep (R, n .. m) : equivalent to R{n, m}

  • Chars "..." : recognize any character in the string

  • Compl R : assume that R is a single-character length regexp (see below) and recognize the complement set

  • Sub (R1,R2) : assume that R is a single-character length regexp (see below) and recognize the set of items in R1 but not in R2 ("subtract")

  • Intersect (R1,R2) : assume that R is a single-character length regexp (see below) and recognize the set of items which are in both R1 and R2

  • lid (lowercase identifier) : reference a named regexp (see below)

A single-character length regexp is a regexp which does not contain (after expansion of references) concatenation, Star, Plus, Opt or string constants with a length different from one.


  • The OCaml source is assumed to be encoded in Latin1 (for string and character literals).

It is possible to define named regular expressions with the following construction, that can appear in place of a structure item:

  let lid = [%sedlex.regexp? R]

where lid is the regexp name to be defined and R its definition. The scope of the "lid" regular expression is the rest of the structure, after the definition.

The same syntax can be used for local binding:

  let lid = [%sedlex.regexp? R] in

The scope of "lid" is the body expression.

Predefined regexps

sedlex provides a set of predefined regexps:

  • any: any character

  • eof: the virtual end-of-file character

  • xml_letter, xml_digit, xml_extender, xml_base_char, xml_ideographic, xml_combining_char, xml_blank: as defined by the XML recommandation

  • tr8876_ident_char: characters names in identifiers from ISO TR8876

  • cc, cf, cn, co, cs, ll, lm, lo, lt, lu, mc, me, mn, nd, nl, no, pc, pd, pe, pf, pi, po, ps, sc, sk, sm, so, zl, zp, zs: as defined by the Unicode standard (categories)

  • alphabetic, ascii_hex_digit, hex_digit, id_continue, id_start, lowercase, math, other_alphabetic, other_lowercase, other_math, other_uppercase, uppercase, white_space, xid_continue, xid_start: as defined by the Unicode standard (properties)

Running a lexer

See the interface of the Sedlexing module for a description of how to create lexbuf values (from strings, stream or channels encoded in Latin1, utf8 or utf16, or from integer arrays or streams representing Unicode code points).

It is possible to work with a custom implementation for lex buffers. To do this, you just have to ensure that a module called Sedlexing is in scope of your lexer specifications, and that it defines at least the following functions: start, next, mark, backtrack. See the interface of the Sedlexing module for more information.

Using sedlex

The quick way:

   opam install sedlex

Otherwise, the first thing to do is to compile and install sedlex. You need a recent version of OCaml and dune.


With findlib

If you have findlib, you can use it to install and use sedlex. The name of the findlib package is "sedlex".

Installation (after "make"):

  make install

Compilation of OCaml files with lexer specifications:

  ocamlfind ocamlc -c -package sedlex.ppx

When linking, you must also include the sedlex package:

  ocamlfind ocamlc -o my_prog -linkpkg -package sedlex.ppx my_file.cmo

There is also a sedlex.ppx subpackage containing the code of the ppx filter. This can be used to build custom drivers (combining several ppx transformations in a single process).

Without findlib

You can use sedlex without findlib. To compile, you need to run the source file through -ppx rewriter ppx_sedlex. Moreover, you need to link the application with the runtime support library for sedlex (sedlexing.cma / sedlexing.cmxa).

With utop

Once sedlex is installed as per above, simply type

#require "sedlex.ppx";;


The examples/ subdirectory contains several samples of sedlex in use.


  • Benus Becker: implementation of Utf16

  • sghost: for Unicode 6.3 categories and properties

  • Peter Zotov:

    • improvements to the build system

    • switched parts of ppx_sedlex to using concrete syntax (with ppx_metaquot)

  • Steffen Smolka: port to dune

Dependencies (5)

  1. uchar
  2. gen
  3. ppxlib >= "0.18.0" & < "0.26.0"
  4. dune >= "2.8"
  5. ocaml >= "4.04" & < "5.0"

Dev Dependencies (1)

  1. odoc with-doc

Used by (35)

  1. acgtk >= "2.0.0"
  2. awsm-codegen
  3. beluga >= "1.0"
  4. catala
  5. chamo >= "3.0"
  6. embedded_ocaml_templates
  7. flow_parser = "0.44.0" | = "0.159.0"
  8. flowtype >= "0.44.0" & < "0.62.0" | >= "0.87.0" & < "0.94.0"
  9. frenetic >= "5.0.0" & != "5.0.3"
  10. fstar >= "2022.01.15"
  11. gufo
  12. haxe >= "4.0.0"
  13. higlo >= "0.8"
  14. iri != "0.4.0"
  15. js_of_ocaml-compiler >= "5.1.0"
  16. kdl
  17. lambdapi >= "2.0.0"
  18. ldp
  19. liquidsoap >= "1.4.0" & < "2.2.0"
  20. m17n
  21. modelica_ml
  22. opazl
  23. owi
  24. passage
  25. pds >= "5.38"
  26. piqilib >= "0.6.15"
  27. ppx_css = "v0.15.1"
  28. ppx_string_interpolation >= "1.0.1"
  29. rdf = "0.10.0" | >= "0.12.0" & < "1.0.0"
  30. scfg
  31. sgf >= "1.0.1"
  32. stk
  33. wcs-lib = "2017-05-26.02" | >= "2017-05-26.05"
  34. xtmpl >= "0.13.0" & < "0.16.0" | >= "0.18.0"
  35. yojson-five




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