@btriller - looking again at this one, wouldn't it be the same to do:
``` subst_hf("X-My-Header", "/^(.+)$/prefix \1/", "a"); subst_hf("X-My-Header", "/^(.+)$/\1 suffix/", "a"); ```
instead of:
``` subst_hf("X-My-Header", "/^/prefix /", "a"); subst_hf("X-My-Header", "/$/ suffix/", "a"); ```