Ocamlnet 4 Reference Manual : Netulex_tut

# Netulex_tut

## Netulex Tutorial

Of course, you need Alain Frisch's `ulex` utility first. It installs itself under the name `ulex` as findlib library.

Next, write your lexer, e.g. (line numbers in brackets):

    [1] open Netulex
    [2] let digits = lexer
    [3]   | ['0'-'9']+ -> `Number(int_of_string(Ulexing.utf8_lexeme lexbuf))
    [4]   | 8364       -> `Euro_sign   (* Code point #8364 in Unicode *)

This is a very trivial example. The lexer accepts sequences of digits, and returns them as `Number tokens. Furthermore, the euro sign is recognized and returned as `Euro_sign. Note that in the first case `Netulex.Ulexing.utf8_lexeme` is called to retrieve the current lexeme as UTF-8 string. (Well, digits are a bad example, as they are only ASCII, and UTF-8 is not really needed. Imagine you want to extend the scanner to other number systems represented in the Unicode character set.)

Line 1 is quite important. If you don't open `Netulex`, the generated `ulex` code will use the version of the `Ulexing` module coming with `ulex`, and not this one.

Call the lexer as follows (line numbers in brackets):

    [5] let sample = "42543\226\130\172";;
    [6] let ulb = Netulex.ULB.from_string `Enc_utf8 sample;;
    [7] let lexbuf = Netulex.Ulexing.from_ulb_lexbuf ulb;;
    [8] let first_token = digits lexbuf;;

Now, `first_token` is ``Number 42543`. After

    [9] let second_token = digits lexbuf;;

this variable is set to ``Euro_sign`, because the three-byte sequence "\226\130\172" represents the euro sign in UTF-8.

In line 6, the encoding ``Enc_utf8` selects that `sample` is an UTF-8 string. You can pass here any encoding the `Netconversion` module understands.

If you would like to scan from another source, just change line 6, e.g.

    [6'] let ulb = Netulex.ULB.from_in_obj_channel ch

where `ch` is any input channel the `Netchannels` module supports. For example, to read from a file:

    let ch = new Netchannels.input_channel (open_in "filename")

You should compile the examples with

    ocamlfind ... -package ulex,netstring -syntax camlp4o ...

For the syntax of the lexer rules, see the documentation coming with `ulex`.