improve jsonic coloring lexer

beautiful-racket/br/demo/jsonic/drracket-lexer.rkt

@@ -6,8 +6,18 @@
   
 (provide drracket-lexer)
 
-(define drracket-lexer
-  (let ([in-racket-expr? #f])
+(define in-racket-expr? #f)
+
+(define (at-racket-boundary? input-port)
+  (equal? (peek-string 2 0 input-port) "$@"))
+
+(module+ test
+  (require rackunit)
+  (check-true (at-racket-boundary? (open-input-string "$@foo")))
+  (check-false (at-racket-boundary? (open-input-string "foo$@"))))
+
+(define (drracket-lexer input-port)
+  (define this-lexer
     (lexer
      [(eof) (values lexeme 'eof #f #f #f)]
      ["@$" (begin
@@ -19,6 +29,7 @@
      [(:seq "//" (:* (char-complement #\newline)))
       (values lexeme 'comment #f (position-offset start-pos) (position-offset end-pos))]
      [any-char
-      (if in-racket-expr?
-          (racket-lexer (transplant-input-port (open-input-string lexeme) #f (position-offset start-pos)))
-          (values lexeme 'string #f (position-offset start-pos) (position-offset end-pos)))])))
+      (values lexeme 'string #f (position-offset start-pos) (position-offset end-pos))]))
+  (if (and in-racket-expr? (not (at-racket-boundary? input-port)))
+      (racket-lexer input-port)
+      (this-lexer input-port)))