Fix another case for expr_match

osscan.cc

@@ -301,10 +301,7 @@ bool expr_match(const char *val, size_t vlen, const char *expr, size_t explen, b
     explen = strlen(expr);
 
   // If both are empty, match; else if either is empty, no match.
-  if (vlen == 0) {
-    return explen == 0;
-  }
-  else if (explen == 0) {
+  if (explen == 0) {
     return vlen == 0;
   }
 
@@ -318,6 +315,20 @@ bool expr_match(const char *val, size_t vlen, const char *expr, size_t explen, b
     q = strchr_p(p, p_end, '|');
     nest = strchr_p(p, q ? q : p_end, '[');
 
+    if (vlen == 0) {
+      // value is empty, so can only match an empty expression
+      if (q == p || p == p_end ) {
+        // expression is also empty, match
+        return true;
+      }
+      else if (!nest) {
+        // simple expression before '|', no match.
+        goto next_expr;
+      }
+      // other short-circuit may be possible here, but drop to nesting logic
+      // below to avoid confusion/bugs
+    }
+
     // if we're already in a nested expr, we skip this and just match as usual.
     if (do_nested && nest) {
       // As long as we keep finding nested portions, e.g. M[>500]ST11W[1-5]

tests/expr_match_test.cc

@@ -182,6 +182,14 @@ const struct expr_test tests[] = {
   {"001", "A", false},
   {"0001", "A", false},
   {"M5B4NNSNW5|M5B4NNSNW7|M5B4NNSNWA", "M5B4NNSNW7", true},
+  {"|U", "U", true},
+  {"|U", "", true},
+  {"|1", "1", true},
+  {"|1", "", true},
+  {"U|", "U", true},
+  {"U|", "", true},
+  {"1|", "1", true},
+  {"1|", "", true},
   {"", "", true}
 };