(testsuite) It's not the empty alternative that's unportable with sed, it's \|.

test/runLint

@@ -17,8 +17,8 @@ gitgrep "\bawk\b.*-F([[:space:]]|[[:space:]]*[\"'][^\"']{2,})" \
 gitgrep '\bsed\b.*\\[?+]' \
     'sed with ? or +, use POSIX BRE instead (\{m,n\})'
 
-gitgrep '\bsed\b.*(\(\\?\||\|\\?\))' \
-    "sed with empty alternative in parens, use '\(...\)\{0,1\}' instead"
+gitgrep '\bsed\b.*\\\|' \
+    "sed with \|, use POSIX BRE (possibly multiple sed invocations) or another tool instead"
 
 # TODO: really nonportable? appears to work fine in Linux, FreeBSD, Solaris
 #gitgrep '\bsed\b.*;' \