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https://github.com/ocaml-community/sedlex

An OCaml lexer generator for Unicode
https://github.com/ocaml-community/sedlex

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An OCaml lexer generator for Unicode

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# sedlex



[![build](https://github.com/ocaml-community/sedlex/actions/workflows/build.yml/badge.svg)](https://github.com/ocaml-community/sedlex/actions/workflows/build.yml)



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.



## API

The API is documented [here](https://ocaml-community.github.io/sedlex).



## 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:



```ocaml

  match%sedlex lexbuf with

  | R1 -> e1

  ...

  | Rn -> en

  | _  -> def

```



or:



```ocaml

  [%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:

- 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.



Note:

 - 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:



```ocaml

  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:



```ocaml

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

  body

```



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](https://dune.build/).



```

  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



- 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