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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 alain.frisch@lexifi.com and is now maintained as part of the ocaml-community repositories on github.

API

The API is documented here.

Overview

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

or:

  [%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.)

where:

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:

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.

Note:

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
  body

The scope of "lid" is the body expression.

Predefined regexps

sedlex provides a set of predefined regexps:

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.

  make

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 my_file.ml

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";;

Examples

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

Contributors