如何在常规语法的模型中实现通配符,字符类,否定字符类等？

Question

TL; DR:如何计算一个语法产品的模型,以便为同一个左手边存在不确定数量的产品？

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我正在研究一个关于形式语言理论的项目,我正在尝试编写一个用于构建常规语法对象的类,这些对象可以传递给有限状态机.我天真的尝试是创建一个API,为每个允许的输入添加一个生产.我的尝试的精简版本如下(基于正式语法的正式定义G = (N, ?, P, S)):

class ContextFreeGrammar:
    def __init__(self, variables, alphabet, production_rules, start_variable):
        self.variables = variables
        self.alphabet = alphabet
        self.production_rules = production_rules
        self.start_variable = start_variable

    def __repr__(self):
        return '{}({}, {}, {}, {})'.format(
            self.__class__.__name__,
            self.variables,
            self.alphabet,
            self.production_rules,
            self.start_variable
        )


class RegularGrammar(ContextFreeGrammar):
    _regular_expression_grammar = None # TODO

    @classmethod
    def from_regular_expression(cls, regular_expression):
        raise NotImplementedError()

我还没有达到实际编写有限状态自动机或下推自动机的程度.

正则表达式的语法是无上下文的,所以我在下面的WSN中包含了我的定义:

syntax = expression .
expression = term "|" expression .
expression = term .
term = factor repetition term .
term = factor term .
term = .
repetition = "*" .
repetition = "+" .
repetition = "?" .
repetition = "{" nonnegative_integer "," nonnegative_integer "}" .
repetition = "{" nonnegative_integer ",}" .
repetition = "{," nonnegative_integer "}" .
nonnegative_integer = nonzero_arabic_numeral arabic_numerals .
nonnegative_integer = arabic_numeral .
nonzero_arabic_numeral = "1" .
nonzero_arabic_numeral = "2" .
nonzero_arabic_numeral = "3" .
nonzero_arabic_numeral = "4" .
nonzero_arabic_numeral = "5" .
nonzero_arabic_numeral = "6" .
nonzero_arabic_numeral = "7" .
nonzero_arabic_numeral = "8" .
nonzero_arabic_numeral = "9" .
arabic_numeral = nonzero_arabic_numeral .
arabic_numeral = "0" .
arabic_numerals = arabic_numeral .
arabic_numerals = arabic_numeral arabic_numerals .
factor = "(" expression ")" .
factor = character_class .
factor = character .
escaped_character = "\\." .
escaped_character = "\\(" .
escaped_character = "\\)" .
escaped_character = "\\+" .
escaped_character = "\\*" .
escaped_character = "\\?" .
escaped_character = "\\[" .
escaped_character = "\\]" .
escaped_character = "\\\\" .
escaped_character = "\\{" .
escaped_character = "\\}" .
escaped_character = "\\|" .
character -> TODO ;
character_class = TODO .

人们可以很容易地看到我明确地将交替分成单独的作品.我这样做是为了便于实施.但我仍然坚持如何进行角色等等.我想要production_rules成为从左手边到每个相应右手边的一套地图.但现在这看起来不可行.

Answer 1

我不完全理解你的问题，但从评论来看，你似乎正在尝试在排除各种 Unicode 和 ASCII 字符的预定义字符集中工作。

这是我最近实现的用于处理类似约束的方法：

[正则表达式] 字符组

这是一个实现上述定义的示例：

global rx_Trim_FromAlphaNumeric
rx_Trim_FromAlphaNumeric =                          \
    "[" + rx_AlphaNumeric                  + "]+" + \
    "[" + rx_ValidCharacters_WithLineSpace + "]*"

global rx_StartsWithSymbol
rx_StartsWithSymbol =                                \
    "[^" + rx_AlphaNumeric                  + "]"  + \
    "["  + rx_Symbols                       + "]+" + \
    "["  + rx_LineSpace + rx_Symbols        + "]*" + \
    "["  + rx_AlphaNumeric                  + "]+" + \
    "["  + rx_ValidCharacters_WithLineSpace + "]*"

global rx_StartsWithLetter
rx_StartsWithLetter =                                \
    "^[" + rx_Alphabetic                    + "]+" + \
    "["  + rx_ValidCharacters_WithLineSpace + "]+"

global rx_StartsWithNumber
rx_StartsWithNumber =                                \
    "^[" + rx_Numeric                       + "]+" + \
    "["  + rx_ValidCharacters_WithLineSpace + "]+"

global rx_WordSegments
rx_WordSegments =                  \
    "([" + rx_Symbols    + "]+|" + \
    "["  + rx_Numeric    + "]+|" + \
    "["  + rx_Alphabetic + "]+|" + \
    "["  + rx_LineSpace  + "]+)"

注意：我更喜欢转义所有符号，因为某些字符（例如 ）^具有上下文转义要求。如果它们总是被逃脱，那么遇到问题的可能性就较小。