Add more checking of variable identifiers

Author: arithy

README.md

@@ -116,6 +116,8 @@ The element stands for the entire pattern in the rule with the name given by _ru
 term <- l:term _ '+' _ r:factor { $$ = l + r; }
 ```
 
+A variable identifier must consist of alphabets (both uppercase and lowercase letters), digits, and underscores. The first letter must be an alphabet. The reserved keywords in C cannot be used.
+
 **_sequence1_ `/` _sequence2_ `/` ... `/` _sequenceN_**
 
 Each _sequence_ is tried in turn until one of them matches, at which time matching for the overall pattern succeeds. If no _sequence_ matches then the matching for the overall pattern fails. The operator slash (`/`) has the least priority. The example is shown below.

src/packcc.c

@@ -1830,6 +1830,11 @@ static node_t *parse_primary(context_t *ctx, node_t *rule) {
             assert(s != VOID_VALUE); /* s should have a valid value when r has a valid value */
             assert(q >= p);
             n_p->data.reference.var = strndup_e(ctx->buffer.buf + p, q - p);
+            if (n_p->data.reference.var[0] == '_') {
+                print_error("%s:%llu:%llu: Leading underscore in variable name '%s'\n",
+                    ctx->iname, (ullong_t)(l + 1), (ullong_t)(m + 1), n_p->data.reference.var);
+                ctx->errnum++;
+            }
             {
                 size_t i;
                 for (i = 0; i < rule->data.rule.vars.len; i++) {