如何为文本输入创建逻辑布尔解析器？

Question

我需要使解析器能够从文本输入中提取逻辑结构,以便为某些Web服务构造查询.

我试图使用正则表达式但处理叠加逻辑变得非常复杂,所以我决定寻求帮助,也许我做错了.

例如:

( (foo1 and bar) or (foo2 and bar2) ) and ( (foo3 and bar3) or foo4 ) and "this is quoted"

结果应该是这样的:

{
    {
        foo1
        AND
        bar
    }
    OR
    {
        foo2
        AND
        bar2
    }
}
AND
{
    {
        foo3
        AND
        bar3
    }
    OR
    foo4
}
AND
{
    "this is quoted"
}

使用的语言是actionscript 3,但我可以调整Java版本.

Answer 1

好吧,解析器很简单......

首先你需要很多东西(我会省略构造函数,因为我猜你可以自己编写):

表达式(输出):

class Expression {}
class Operation extends Expression {
    public var operand1:Expression;
    public var operator:String;
    public var operand2:Expression;
}
class Atom extends Expression {
    public var ident:String;
}

令牌(中介格式):

class Token {
    public var source:String;
    public var pos:uint;
}
class Identiefier extends Token {
    public var ident:String;
}
class OpenParenthesis extends Token {}
class CloseParenthesis extends Token {}
class Operator extends Token {
    public var operator:String;
}
class Eof extends Token {}

和一个应该实现此接口的标记化器

interface TokenStream {
    function read():Token;
}

我猜你会弄清楚如何标记......

所以方法是源 - (tokenizer) - >令牌 - (解析器) - >表达式......

在这里解析例程,带一个小助手:

function parse(t:TokenStream):Expression {
    var tk:Token = t.read();
    switch ((tk as Object).constructor) {//this is a really weird thing about AS3 ... need to cast to object, before you can access the constructor
        case OpenParanthesis:
            var e1:Expression = parse(t);
            tk = t.read();
            switch ((tk as Object).constructor) {
                case CloseParenthesis:
                    return e1;
                case Operator:
                    var op:String = (tk as Operator).operator;
                    var e2:Expression = parse(t);
                    tk = t.read();
                    if (tk is CloseParenthesis)
                        return new Operation(e1,op,e2);
                    else
                        unexpected(tk);
            }
            else
                unexpected(tk);
        break;
        case Identifier:
            return new Atom((tk as Identifier).ident);
        default:
            unexpected(tk);
    }
}
function unexpected(tk:Token) {
    throw "unexpected token "+tk.source+" at position "+tk.pos;
}

这不是一个特别好的解析器,但它显示了解析例程的基本原理......实际上,我没有检查实现,但它应该工作......它是非常原始的和不允许的......像运算符优先级等完全缺失,等等...但是如果你想要它,那就去吧...

顺便说一句.将haXe与enum一起使用,整个代码看起来会更短更漂亮......你可能想看看它......

那么祝你好运吧 ... ;)

格尔茨

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