如何在规则语法模型中实现通配符、字符类、否定字符类等？

Question

TL;DR：

如何在计算上模拟语法的结果，使同一左手边存在不确定数量的产品？

我正在从事一个关于形式语言理论的项目，并试图编写一个类来构建规则语法对象，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，用于为每个允许的输入添加一个产品。我尝试的简化版本如下(基于正式语法G = (N, Σ, P, S)的正式定义)：

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()

我还没有到实际写有限状态自动机或下推自动机的地步。

正则表达式的语法是上下文无关的，因此我在WSN中包含了我的定义如下：

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，我显式地将交替分割成不同的产品。我这样做是为了便于执行。但是，我被困在如何去做角色类之类的事情上了。我想让production_rules成为一个地图，从每个左手侧到每一个对应的右手边的一组地图。但现在看来不可行了。

Answer 1

我不完全理解您的问题，但从注释中可以看出，您似乎是在预定义的字符集内工作，该字符集不包括杂项Unicode和ASCII字符。

下面是我最近实现的用于处理类似约束的方法：

[RegEx]字符组

下面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:我喜欢转义所有符号，因为某些字符，例如^__，有上下文转义的要求。如果他们总是逃脱，就不太可能遇到问题。