diff --git a/pkgs/build-support/fetchurl/write-mirror-list.sh b/pkgs/build-support/fetchurl/write-mirror-list.sh
index 55508742dd3..2dabd2e722b 100644
--- a/pkgs/build-support/fetchurl/write-mirror-list.sh
+++ b/pkgs/build-support/fetchurl/write-mirror-list.sh
@@ -1,4 +1,4 @@
 source $stdenv/setup
 
 # !!! this is kinda hacky.
-set | grep '^[a-zA-Z]\+=.*://' > $out
+set | grep -E '^[a-zA-Z]+=.*://' > $out