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diff --git a/grammar.scm b/grammar.scm
index 4778283..6bbc371 100644
--- a/grammar.scm
+++ b/grammar.scm
@@ -140,16 +140,14 @@

 ;; char    <- #\\ ( escape / unicode ) / (![[:cntrl:]"\\] .)

-(define char-set:json-special-char
-  (char-set-union
-   (ucs-range->char-set #x0 #x20)
+(define char-set:json-valid-char
+  (char-set-difference
+   char-set:full
+   char-set:iso-control
    (char-set #\" #\\)))

 (define char
-  (any-of (char-seq-match '(: bos (+ (/ #\x20 #\x21
-                                        #\x23 #\x5b
-                                        #\x5d #\x7f))))
-          (none-of* (in char-set:json-special-char) utf8-char)
+  (any-of (in char-set:json-valid-char)
           (preceded-by (is #\\) (any-of escape unicode fail))))

 ;; raw-string <- #\" char* #\"