问题描述
我不知道如何执行此操作，而且我没有在线上找到有关如何执行此操作的好资源[.]我正在尝试采用带注释的EBNF生产规则，该规则是两个正则表达式之间的区别并把它变成一个(na | f?)lex语法规范规则[.]问题是我看不到通常能做到这一点的方法[.] {3}，有一种方法可以像克莱恩代数一样使用克莱恩代数来做到这一点.您可以在上下文无关的语法中使用带交替符的空匹配[?]
I don't know how to do this, and I've found no good resources online for how to perform this operation[.] I'm trying to take an annotated EBNF production rule which is a difference between two regular expressions and turn it into a(n a| f?)lex grammar specification rule[.] The problem is that I see no way to do this normally[.]{3} is there a way to do this using Kleene algebra, like the way you can use an empty match with alternation in a context-free grammar[?]
推荐答案
EBNF生产规则是什么样的(您可以在EBNF中写出来吗?)
What does the EBNF production rule look like (and can you write that in EBNF?)
正则表达式之间的集合差异A\B等于与补码的交集:A&~B:与A匹配的字符串集，没有与B匹配的任何字符串.flex和alex都不在其正则表达式中支持此类语义表达语言.
The set difference A\B between regular expressions is equivalent to intersection with a complement: A&~B: the set of strings matched by A, devoid of any strings matched by B. Neither flex nor alex support such semantics in their regular expression languages.
这种形式的表达式仍表示正则语言，因此具有等效的自动机，为此，仅基于交替和Kleene运算符才存在正则表达式.这种等效表达式不容易找到，并且取决于A和B的特定内部.也就是说，我们不能仅将A和B插入一些通用的正则表达式中，以使结果表示集合差异.原始A和B将不会出现在设置差异正则表达式中.
An expression of this form still denotes a regular language, and therefore has an equivalent automaton, for which a regular expression exists based only on the alternation and Kleene operators. This equivalent expression not easy to find, and it depends on the specific internals of A and B. That is to say, we can't just plug A and B into some general regex formula such that the result denotes the set difference. The original A and B will not appear in the set difference regex.
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