disallow operators starting with a closing bracket

src/pratt_parser.c

@@ -620,6 +620,8 @@ static void validateOperator(PrattParser *parser, PrattUTF8 *operator) {
         parserError(parser, "operator cannot start with a numeric digit");
     } else if (utf8_isopen(operator->entries)) {
         parserError(parser, "operator cannot start with an opening bracket");
+    } else if (utf8_isclose(operator->entries)) {
+        parserError(parser, "operator cannot start with a closing bracket");
     } else {
         for (Index i = 0; i < operator->size; i++) {
             if (isspace(operator->entries[i])) {

src/unicode.c

@@ -72,6 +72,10 @@ bool unicode_isopen(Character c) {
     return unicode_isvalid(c) && (category[c] == GC_Ps);
 }
 
+bool unicode_isclose(Character c) {
+    return unicode_isvalid(c) && (category[c] == GC_Pe);
+}
+
 bool unicode_issymbol(Character c) {
     return unicode_isvalid(c) && ((category[c] & GC_MASK) == GC_S);
 }

src/unicode.h

@@ -74,6 +74,7 @@ bool unicode_isalnum(Character c);
 bool unicode_isalpha(Character c);
 bool unicode_isascii(Character c);
 bool unicode_isblank(Character c);
+bool unicode_isclose(Character c);
 bool unicode_iscntrl(Character c);
 bool unicode_isdigit(Character c);
 bool unicode_isgraph(Character c);

src/utf8.c

@@ -431,3 +431,9 @@ bool utf8_isopen(unsigned char *s) {
     return unicode_isopen(c);
 }
 
+bool utf8_isclose(unsigned char *s) {
+    Character c = 0;
+    utf8Sgetc(s, &c);
+    return unicode_isclose(c);
+}
+

src/utf8.h

@@ -69,6 +69,7 @@ bool utf8_isalnum(unsigned char *s);
 bool utf8_isalpha(unsigned char *s);
 bool utf8_isascii(unsigned char *s);
 bool utf8_isblank(unsigned char *s);
+bool utf8_isclose(unsigned char *s);
 bool utf8_iscntrl(unsigned char *s);
 bool utf8_isdigit(unsigned char *s);
 bool utf8_isgraph(unsigned char *s);