Don't recompile the same regex over and over

src/libexpr/names.cc

@@ -1,6 +1,5 @@
 #include "names.hh"
 #include "util.hh"
-#include "regex.hh"
 
 
 namespace nix {
@@ -34,8 +33,8 @@ DrvName::DrvName(const string & s) : hits(0)
 bool DrvName::matches(DrvName & n)
 {
     if (name != "*") {
-        Regex regex(name);
+        if (!regex) regex = std::shared_ptr<Regex>(new Regex(name));
-        if (!regex.matches(n.name)) return false;
+        if (!regex->matches(n.name)) return false;
     }
     if (version != "" && version != n.version) return false;
     return true;

src/libexpr/names.hh

@@ -1,6 +1,9 @@
 #pragma once
 
+#include <memory>
+
 #include "types.hh"
+#include "regex.hh"
 
 namespace nix {
 
@@ -14,6 +17,9 @@ struct DrvName
     DrvName();
     DrvName(const string & s);
     bool matches(DrvName & n);
+
+private:
+    std::shared_ptr<Regex> regex;
 };
 
 typedef list<DrvName> DrvNames;