我试图找出是否可能有一个CFG的例子,因为它不可能给出一个可以接受相同语言的正则表达式。
Since a regular machine/expression has only a limited (pre-defined) number of states, it cannot "remember" (infinitely) earlier parts of the input.
As such recognizing the following expression is impossible for a state-machine: a n b n (n∈ℕ)
You could make such a machine for n ≤ x, where x∈ℕ, but no state-machine can do it for every possible value from ℕ.
Any language which requires counting/remembering can't be expressed as a regular expression.
For example, a language which checks balanced parenthesis:
S -> { S } S
S -> ε
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