(** * Definitions *)
(** [RegExp], a type for regular expressions, consists of following constructors: - [Empty] : matches no strings, - [Eps] : matches an empty string $(\epsilon)$, - [Char c] : matches a single charater [c], - [Cat r1 r2] : [r1 ++ r2] matches [s1 ++ s2] iff [r1, r2] match [s1, s2], respectively, - [Or r1 r2] : [r1 || r2] matches [s1] or [s2] iff [r1, r2] match [s1, s2], respectively, - [Star r] : [Star r] matches a zero-or-more times repetition of [s] iff [r] matches [s]; Kleene star $r^{\ast}$ #r*#, - [Not r] : [Not r] matches [s] iff [r] does not match [s], - [And r1 r2] : [And r1 r2] matches [s] iff both [r1, r2] match [s].  Though [Not] and [And] are not necessary for regular expression, they would be useful in real use.  You may add another constructor to [RegExp] because no inductions on [RegExp] are used in the proofs of the library. Just add your constructor to definitions of [RegExp], [nu], and [derive] with consistency. *)
(** * Coq codes *)
(** ** Dependencies *)
Require Export RegExp.Utility.
Require Export Setoid.
Require Export Relation_Definitions.
(** ** Definitions *)
Inductive RegExp : Set :=
| Empty : RegExp
| Eps : RegExp
| Char : ascii -> RegExp
| Cat : RegExp -> RegExp -> RegExp
| Or : RegExp -> RegExp -> RegExp
| Star : RegExp -> RegExp
| Not : RegExp -> RegExp
| And : RegExp -> RegExp -> RegExp.
Notation "a ++ b" := (Cat a b).
Notation "a || b" := (Or a b).
(** [nu re = true] iff [re:RegExp] accepts empty string. *)
Fixpoint nu(re:RegExp):bool :=
match re with
| Empty => false
| Eps => true
| Char c => false
| Cat r s => (nu r && nu s)%bool
| Or r s => (nu r || nu s)%bool
| Star r => true
| Not r => negb (nu r)
| And r s => (nu r && nu s)%bool
end.
(** Derivation of [re:RegExp] by [a:ascii]. *)
Fixpoint derive(a:ascii)(re:RegExp):RegExp :=
match re with
| Empty => Empty
| Eps => Empty
| Char c => match (ascii_dec c a) with
| left _ => Eps
| right _ => Empty
end
| Cat r s => match (nu r) with
| true => ((derive a r) ++ s) || (derive a s)
| false => (derive a r) ++ s
end
| Or r s => (derive a r) || (derive a s)
| Star r => (derive a r) ++ (Star r)
| Not r => Not (derive a r)
| And r s => And (derive a r) (derive a s)
end.
Notation "re / a" := (derive a re).
(** [matches re s = true] iff [re:RegExp] matches [s:string]. *)
Fixpoint matches (re:RegExp)(s:string) : bool :=
match s with
| EmptyString => nu re
| String a w => matches (re / a) w
end.
Notation "re ~= s" := (matches re s) (at level 60).
Notation "re ~== s" := (matches re s = true) (at level 60).
Notation "re ~!= s" := (matches re s = false) (at level 60).
(** Relation between [matches] and [derive]. *)

(* Specifications: *)
Theorem derivation : forall (a:ascii)(w:string)(re:RegExp),
re ~= (String a w) = (re / a) ~= w.