The rules for a lexer are bound to be all of the same standard pattern:

rule FirstDigit
    salience 0
    where
        $i : Input( $ch : character matches "\d" )
        $l : Lexer( state == State.IDLE, $buf : buffer )
    then
        $buf.setLength( 0 ); $buf.append( $ch );
        modify( $l ){ setState( State.NUMBER ) }
        modify( $i ){ next() }
end

Rather boring.

If you prefer, you can write a single rule exercising the FSM mechanism (match input, execute action, change state) that is driven by a set of (static and immutable) facts defining the individual state/symbol/action/transition quadruples.

rule Lex
    salience 0
    when
        $k : CharClass( $pattern : pattern )                  // such as "\d" or "\w"
        $i : Input( $ch : character matches $pattern )
        $l : Lexer( $state : state )                                  // also containing a StringBuilder buffer
        $t : StateTransition( state == $state, charClass == $k, $action : action, $succ : succ )
    then
        $action.execute( $i );
        modify( $l ){ setState( $succ ) }
        modify( $i ){ next() }
end

An additional rule catches errors. - An advantage of this approach is that any lexer can now be configured by a set of fact definitions, e.g., this one for a sequence of identifiers, numbers and quoted strings:

        CharClass letter = new CharClass( "[A-Za-z_]" );
        CharClass digit = new CharClass( "\\d" );
        CharClass escape = new CharClass( "\\\\" );
        CharClass space = new CharClass( "\\s" );
        CharClass quote = new CharClass( "\"" );
        CharClass empty = new CharClass( "\u0000" );
        ins( letter, digit, escape, space, quote, empty );

        Action<State,TokenType> save = new SaveAction( this );
        Action<State,TokenType> skip = new SkipAction( this );
        Action<State,TokenType> halt = new HaltAction( this );
        Action<State,TokenType> emit = new EmitAction( this );

        StateTransition<State,TokenType> t1 = new StateTransition<State,TokenType>( State.S0, letter, save, State.IDENT );
        StateTransition<State,TokenType> t2 = new StateTransition<State,TokenType>( State.S0, digit, save, State.NUMBER );
        StateTransition<State,TokenType> t3 = new StateTransition<State,TokenType>( State.S0, quote, skip, State.STRING );
        StateTransition<State,TokenType> t4 = new StateTransition<State,TokenType>( State.S0, space, skip );
        StateTransition<State,TokenType> t5 = new StateTransition<State,TokenType>( State.S0, empty, halt );
        ins( t1, t2, t3, t4, t5 );

        StateTransition<State,TokenType> u1 = new StateTransition<State,TokenType>( State.IDENT, letter, save );
        StateTransition<State,TokenType> u2 = new StateTransition<State,TokenType>( State.IDENT, digit, save );
        StateTransition<State,TokenType> u3 = new StateTransition<State,TokenType>( State.IDENT, space, emit, State.S0 );
        StateTransition<State,TokenType> u4 = new StateTransition<State,TokenType>( State.IDENT, empty, halt );
        ins( u1, u2, u3, u4 );

        StateTransition<State,TokenType> v1 = new StateTransition<State,TokenType>( State.NUMBER, digit, save );
        StateTransition<State,TokenType> v2 = new StateTransition<State,TokenType>( State.NUMBER, space, emit, State.S0 );
        StateTransition<State,TokenType> v3 = new StateTransition<State,TokenType>( State.NUMBER, empty, halt );
        ins( v1, v2, v3 );

        StateTransition<State,TokenType> w1 = new StateTransition<State,TokenType>( State.STRING, letter, save );
        StateTransition<State,TokenType> w2 = new StateTransition<State,TokenType>( State.STRING, digit, save );
        StateTransition<State,TokenType> w3 = new StateTransition<State,TokenType>( State.STRING, space, save );
        StateTransition<State,TokenType> w4 = new StateTransition<State,TokenType>( State.STRING, quote, emit, State.S0 );
        StateTransition<State,TokenType> w5 = new StateTransition<State,TokenType>( State.STRING, escape, save, State.ESCAPE );
        StateTransition<State,TokenType> w6 = new StateTransition<State,TokenType>( State.STRING, empty, halt );
        ins( w1, w2, w3, w4, w5, w6 );

        StateTransition<State,TokenType> x1 = new StateTransition<State,TokenType>( State.ESCAPE, letter, save, State.STRING );
        StateTransition<State,TokenType> x2 = new StateTransition<State,TokenType>( State.ESCAPE, digit, save, State.STRING );
        StateTransition<State,TokenType> x3 = new StateTransition<State,TokenType>( State.ESCAPE, space, save, State.STRING );
        StateTransition<State,TokenType> x4 = new StateTransition<State,TokenType>( State.ESCAPE, quote, emit, State.STRING );
        StateTransition<State,TokenType> x5 = new StateTransition<State,TokenType>( State.ESCAPE, escape, save, State.STRING );
        StateTransition<State,TokenType> x6 = new StateTransition<State,TokenType>( State.ESCAPE, empty, halt );
        ins( x1, x2, x3, x4, x5, x6 );

-W

On Wed, Sep 2, 2009 at 11:54 PM, André Thieme <address.good.until.2009.dec.14@justmail.de> wrote:

Hello group, I recently had the idea:
"A rule system (like Drools) is ideal for making programs with complex
rules simpler. Writing a lexer or parser can be non-trivial. So, is it
possible and also meaningful to express such a task with rules?"

Anyone here who maybe tried that already?
The two big questions for me are:
1) how easy is it to express a lexer with rules?
2) how bad (good?) will it perform?

If you happen to have a good idea of how to do it, could you please give
me an example for a simple lexer?
Let's say it will get natural language (a string, such as this email) as
input and should return a sequence (say, ArrayList) of Tokens, which may
look like this:

public class Token {
public String value;
public String category;

Token(String value, String category) {
this.value = value;
this.category = category;
}
}

We could have three categories:
"word", "numeric" and "whitespace".

An input String could be:
"We can see 500 cars"
And it should produce an ArrayList with the contents:
[
Token("We", "word"),
Token(" ", "whitespace"),
Token("can", "word"),
Token(" ", "whitespace"),
Token("see", "word"),
Token(" ", "whitespace"),
Token("500", "numeric"),
Token(" ", "whitespace"),
Token("cars", "word")
]

At the moment I have difficulties to see if/how this could be achieved.
If you find this easy, please post a solution.
I am aware that JavaCC is really good for such tasks and will also
perform extremly well.

Greetings,
André
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