Question

嗨，我的正则表达式具有以下数据类型：

data RE a            -- regular expressions over an alphabet defined by 'a'
= Empty          -- empty regular expression
| Sym a          -- match the given symbol
| RE a :+: RE a  -- concatenation of two regular expressions
| RE a :|: RE a  -- choice between two regular expressions
| Rep (RE a)     -- zero or more repetitions of a regular expression
| Rep1 (RE a)    -- one or more repetitions of a regular expression
deriving (Show)

，我需要设计一种方法来检查该正则表达式是否包含Empty字符串。例如（Rep（Sym'a'：+：Sym'b'））为true，而（Sym'a'）为false。我将如何使该方法返回布尔值？

Answer 1

计算Salomaa的“空词”属性（有时也称为“可空”）通常是通过将正则表达式放入正则表达式中来完成的，该正则表达式利用代数属性来删除多余的Kleene星，然后检查是否还有星运算符表达式，或者表达式是否具有或本身就是您定义的Empty。这是example。

-- decide if the language defined by r contains ε, i.e.
-- nullable (r) ⇔ ε ∈ ℒ(r)
-- Also know as Salomaa's Empty Word Property (EWP)
nullable ∷ (Ord s) ⇒ RegExp s → Bool
nullable = nullable' . normalize
  where nullable' Zero     = False
        nullable' One      = True
        nullable' (Lit  _) = False
        nullable' (α :| β) = nullable' α || nullable' β
        nullable' (α :. β) = nullable' α && nullable' β
        nullable' (Star _) = True

为Rep1添加一个保护套看起来像nullable' (Rep1 α) = nullable' α。

Haskell函数检查正则表达式是否为空

1 个答案: