diff --git a/spa/include/spa/utils/json-core.h b/spa/include/spa/utils/json-core.h
index 1a82df4bf..0ffb21abf 100644
--- a/spa/include/spa/utils/json-core.h
+++ b/spa/include/spa/utils/json-core.h
@@ -488,14 +488,20 @@ SPA_API_JSON bool spa_json_is_json_number(const char *val, int len)
 	/* 7 */ { 3,   3,    8,   8,  -1,  -1,   -1 },  /* after 'e'/'E' */
 	/* 8 */ { 3,   3,   -1,  -1,  -1,  -1,   -1 },  /* after exp sign */
 	};
+	static const int8_t char_class[128] = {
+		6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,  /* 0-15 */
+		6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,  /* 16-31 */
+		6,6,6,6,6,6,6,6,6,6,6,3,6,2,4,6,  /* 32-47: + - . */
+		1,0,0,0,0,0,0,0,0,0,6,6,6,6,6,6,  /* 48-63: 0-9 */
+		6,6,6,6,6,5,6,6,6,6,6,6,6,6,6,6,  /* 64-79: E */
+		6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,  /* 80-95 */
+		6,6,6,6,6,5,6,6,6,6,6,6,6,6,6,6,  /* 96-111: e */
+		6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,  /* 112-127 */
+	};
 	int i, state = 4;
 
 	for (i = 0; i < len; i++) {
-		char v = val[i];
-		int cls = (v >= '1' && v <= '9') ? 0 : v == '0' ? 1 :
-		          v == '-' ? 2 : v == '+' ? 3 : v == '.' ? 4 :
-		          (v == 'e' || v == 'E') ? 5 : 6;
-		if ((state = trans[state][cls]) < 0)
+		if ((state = trans[state][char_class[val[i]&0x7f]]) < 0)
 			return false;
 	}
 	return state < 4;