+# Filter a list by regexp.
+pure function filter (pat, xs) returning strings = @{
+ for f in %xs; do echo "$f"; done | grep -E -- %pat
+}
+
+# Filter out a list by regexp.
+pure function filter-out (pat, xs) returning strings = @{
+ for f in %xs; do echo "$f"; done | grep -v -E -- %pat
+}
+