permit ::= as rule separator

brag/brag/rules/lexer.rkt

@@ -30,6 +30,7 @@
   )
 
 (define-lex-abbrev id (:& (complement (:+ digit)) (:+ id-char)))
+(define-lex-abbrev id-separator (:or ":" "::="))
 
 (define lex/1
   (lexer-src-pos
@@ -76,11 +77,11 @@
     (return-without-pos (lex/1 input-port))]
    [(eof)
     (token-EOF lexeme)]
-   [(:: id (:* whitespace) ":")
+   [(:: id (:* whitespace) id-separator)
     (token-RULE_HEAD lexeme)]
-   [(:: hide-char id (:* whitespace) ":")
+   [(:: hide-char id (:* whitespace) id-separator)
     (token-RULE_HEAD_HIDDEN lexeme)]
-   [(:: splice-char id (:* whitespace) ":")
+   [(:: splice-char id (:* whitespace) id-separator)
     (token-RULE_HEAD_SPLICED lexeme)]
    [id
     (token-ID lexeme)]

brag/brag/rules/parser.rkt

@@ -59,6 +59,8 @@
 (define hide-char #\/)
 (define splice-char #\@)
 
+(define id-separators-pattern "((:)|(::=))")
+
 ;; grammar-parser: (-> token) -> (listof rule)
 (define grammar-parser
   (parser
@@ -84,7 +86,7 @@
     [rule
      [(RULE_HEAD pattern)
       (begin 
-        (define trimmed (regexp-replace #px"\\s*:$" $1 ""))
+        (define trimmed (regexp-replace (pregexp (format "\\s*~a$" id-separators-pattern)) $1 ""))
         (rule (position->pos $1-start-pos)
               (position->pos $2-end-pos)
               (lhs-id (position->pos $1-start-pos)
@@ -98,7 +100,7 @@
      
      [(RULE_HEAD_HIDDEN pattern) ; slash indicates hiding
       (begin
-        (define trimmed (cadr (regexp-match (pregexp (format "~a(\\S+)\\s*:$" hide-char)) $1)))
+        (define trimmed (cadr (regexp-match (pregexp (format "~a(\\S+)\\s*~a$" hide-char id-separators-pattern)) $1)))
         (rule (position->pos $1-start-pos)
               (position->pos $2-end-pos)
               (lhs-id (position->pos $1-start-pos)
@@ -113,7 +115,7 @@
      
      [(RULE_HEAD_SPLICED pattern) ; atsign indicates splicing
       (begin
-        (define trimmed (cadr (regexp-match (pregexp (format "~a(\\S+)\\s*:$" splice-char)) $1)))
+        (define trimmed (cadr (regexp-match (pregexp (format "~a(\\S+)\\s*~a$" splice-char id-separators-pattern)) $1)))
         (rule (position->pos $1-start-pos)
               (position->pos $2-end-pos)
               (lhs-id (position->pos $1-start-pos)