Return an empty view for a non-participating capture group

src/tsutil/Regex.cc

@@ -205,6 +205,13 @@ RegexMatches::operator[](size_t index) const
   }
 
   PCRE2_SIZE *ovector = pcre2_get_ovector_pointer(_MatchData::get(_match_data));
+
+  // A group that did not participate in the match has an unset offset. This happens for an optional
+  // group that precedes a participating one, so a valid index is not enough to guarantee an offset.
+  if (PCRE2_UNSET == ovector[2 * index]) {
+    return std::string_view();
+  }
+
   return std::string_view(_subject.data() + ovector[2 * index], ovector[2 * index + 1] - ovector[2 * index]);
 }
 

src/tsutil/unit_tests/test_Regex.cc

@@ -460,6 +460,23 @@ TEST_CASE("RegexMatches edge cases", "[libts][Regex][RegexMatches]")
     CHECK(count >= 2); // At least whole match + first group
     CHECK(matches[1] == "foo");
   }
+
+  SECTION("RegexMatches with a non-participating group before a participating one")
+  {
+    // pcre2_match() returns one past the highest participating group, so an earlier optional group
+    // that did not participate is still within that count. Its offsets are unset.
+    Regex r;
+    REQUIRE(r.compile("(a)?(b)") == true);
+
+    RegexMatches matches;
+    int          count = r.exec("b", matches);
+
+    CHECK(count == 3);
+    CHECK(matches[0] == "b");
+    CHECK(matches[1] == "");
+    CHECK(matches[2] == "b");
+    CHECK(matches[1].data() == nullptr);
+  }
 }
 
 TEST_CASE("Regex with special characters", "[libts][Regex][special]")