exploring grammars

1.ebnf

(* as checked here: https://mdkrajnak.github.io/ebnftest/ *)
(* ebnf syntax rules defined here: https://github.com/Engelberg/instaparse *)

Program                 := WhSp* (StmtSemi WhSp*)* StmtSemiOpt? WhSp*;

Stmt                    := AStmt | BStmt | CStmt | DStmt;
StmtSemi                := Stmt? (WhSp* ";")+;
StmtSemiOpt             := Stmt? (WhSp* ";")*;

WhSp                    := "_";

AStmt                   := "a";
BStmt                   := "b";
CStmt                   := "c";
DStmt                   := "d";

1b.ebnf

(* as checked here: https://mdkrajnak.github.io/ebnftest/ *)
(* ebnf syntax rules defined here: https://github.com/Engelberg/instaparse *)
(* backport from the normalized bnf in '2b.bnf' *)

Leading                 := WhSp Leading Program | Semi Leading Program | Program;
Program                 := Stmt | Stmt WhSp | Stmt WhSpOptSemi Program | Stmt WhSpOptSemi WhSp Program | Epsilon;

Stmt                    := AStmt | BStmt | CStmt | DStmt;

WhSpOptSemi             := Semi | WhSp Semi | Semi WhSpOptSemi | WhSp Semi WhSpOptSemi;
WhSp                    := "_" | "_" WhSp;
Semi                    := ";";

AStmt                   := "a";
BStmt                   := "b";
CStmt                   := "c";
DStmt                   := "d";

2.bnf

/* as checked here: https://smlweb.cpsc.ucalgary.ca/start.html */
/* analysis here: shorturl.at/bjBO7 */
/* bnf syntax rules defined here: https://smlweb.cpsc.ucalgary.ca/readme.html */

Program                 -> StmtSemi FinalStmtSemiOpt .
StmtSemi                -> WhSp StmtSemiOpt | StmtSemiOpt .
FinalStmtSemiOpt        -> StmtOpt SemiOpt WhSpOpt | WhSpOpt .

Stmt                    -> AStmt | BStmt | CStmt | DStmt .
StmtOpt                 -> Stmt | .
StmtSemiOpt             -> StmtOpt Semi | StmtOpt Semi WhSpOpt StmtSemiOpt | .

Semi                    -> WhSpOpt ; | WhSpOpt ; Semi .
SemiOpt                 -> Semi | .

WhSp                    -> _ | _ WhSp .
WhSpOpt                 -> WhSp | .

AStmt                   -> a .
BStmt                   -> b .
CStmt                   -> c .
DStmt                   -> d .

2b.bnf

/* as checked here: https://smlweb.cpsc.ucalgary.ca/start.html */
/* analysis here: shorturl.at/huCOP */
/* bnf syntax rules defined here: https://smlweb.cpsc.ucalgary.ca/readme.html */

Leading                 -> WhSp Leading Program | Semi Leading Program | Program .
Program                 -> Stmt | Stmt WhSp | Stmt WhSpOptSemi Program | Stmt WhSpOptSemi WhSp Program | .

Stmt                    -> AStmt | BStmt | CStmt | DStmt .

WhSpOptSemi             -> Semi | WhSp Semi | Semi WhSpOptSemi | WhSp Semi WhSpOptSemi .
WhSp                    -> _ | _ WhSp .
Semi                    -> ; .

AStmt                   -> a .
BStmt                   -> b .
CStmt                   -> c .
DStmt                   -> d .

3.valid.txt

_____
;;;;;
_;_;_
a
__a__
a;
a;b;
a;_b;
_a;_b;_
_a_;_b_;_
__a__;;
_;_a;_b;c;;;__;;__d;___a___

4.invalid.txt

ab
a_b
a;_b_c