diff --git a/Source/kwsys/RegularExpression.cxx b/Source/kwsys/RegularExpression.cxx
index 89cab6c81..a1c98eaef 100644
--- a/Source/kwsys/RegularExpression.cxx
+++ b/Source/kwsys/RegularExpression.cxx
@@ -348,7 +348,7 @@ static int strcspn ();
 bool RegularExpression::compile (const char* exp) {
     register const char* scan;
     register const char* longest;
-    register unsigned long len;
+    register size_t len;
              int         flags;
 
     if (exp == 0) {