diff --git a/include/sta/PatternMatch.hh b/include/sta/PatternMatch.hh
index b08dcb21..0604d4f4 100644
--- a/include/sta/PatternMatch.hh
+++ b/include/sta/PatternMatch.hh
@@ -44,7 +44,7 @@ public:
   PatternMatch(const char *pattern);
   PatternMatch(const char *pattern,
 	       const PatternMatch *inherit_from);
-  PatternMatch(string pattern,
+  PatternMatch(const string &pattern,
 	       const PatternMatch *inherit_from);
   bool match(const char *str) const;
   bool match(const string &str) const;
diff --git a/util/PatternMatch.cc b/util/PatternMatch.cc
index 8ce8bb71..1202c369 100644
--- a/util/PatternMatch.cc
+++ b/util/PatternMatch.cc
@@ -57,7 +57,7 @@ PatternMatch::PatternMatch(const char *pattern,
     compileRegexp();
 }
 
-PatternMatch::PatternMatch(string pattern,
+PatternMatch::PatternMatch(const string &pattern,
 			   const PatternMatch *inherit_from) :
   pattern_(pattern.c_str()),
   is_regexp_(inherit_from->is_regexp_),