diff --git a/builder/default.nix b/builder/default.nix
index 731b415..ac06757 100644
--- a/builder/default.nix
+++ b/builder/default.nix
@@ -261,13 +261,20 @@ let
         buildPhase = attrs.buildPhase or ''
           runHook preBuild
 
+          exclude='\(/_\|examples\|Godeps\|testdata'
+          if [[ -n "$excludedPackages" ]]; then
+            IFS=' ' read -r -a excludedArr <<<$excludedPackages
+            printf -v excludedAlternates '%s\\|' "''${excludedArr[@]}"
+            excludedAlternates=''${excludedAlternates%\\|} # drop final \| added by printf
+            exclude+='\\|'"$excludedAlternates"
+          fi
+          exclude+='\)'
+
           buildGoDir() {
             local d; local cmd;
             cmd="$1"
             d="$2"
             . $TMPDIR/buildFlagsArray
-            echo "$d" | grep -q "\(/_\|examples\|Godeps\|testdata\)" && return 0
-            [ -n "$excludedPackages" ] && echo "$d" | grep -q "$excludedPackages" && return 0
             local OUT
             if ! OUT="$(go $cmd $buildFlags "''${buildFlagsArray[@]}" ''${tags:+-tags=${lib.concatStringsSep "," tags}} ''${ldflags:+-ldflags="$ldflags"} -v -p $NIX_BUILD_CORES $d 2>&1)"; then
               if echo "$OUT" | grep -qE 'imports .*?: no Go files in'; then
@@ -308,6 +315,7 @@ let
               export NIX_BUILD_CORES=1
           fi
           for pkg in $(getGoDirs ""); do
+            grep -q "$exclude" <<<$pkg && continue
             echo "Building subPackage $pkg"
             buildGoDir install "$pkg"
           done