diff --git a/src/ext/xregex.h b/src/ext/xregex.h
index 7e1c75f..ac734ec 100644
--- a/src/ext/xregex.h
+++ b/src/ext/xregex.h
@@ -16,7 +16,7 @@
 
 #include <regex.h>
 
-#define XREGEX_IS_MATCHGROUP(groupArray, groupIndex) (groupArray[groupIndex].rm_so < groupArray[groupIndex].rm_eo)
+#define XREGEX_IS_MATCHGROUP(groupArray, groupIndex) (groupArray[groupIndex].rm_so < groupArray[groupIndex].rm_eo && 0 <= groupArray[groupIndex].rm_so)
 #define XREGEX_SIZEOF_MATCHGROUP(groupArray, groupIndex) (size_t) (groupArray[groupIndex].rm_eo - groupArray[groupIndex].rm_so)
 
 #endif /* EXT_XREGEX_H_ */