Validate regular expressions before reading input

scripts/test-pipe-regex-fallback.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+set -eu
+
+sed_bin=$1
+
+# Limine uses this portability probe in gensyms.sh.  The first sed must reject
+# the unsupported word-boundary character classes without consuming the pipe,
+# leaving the input available to the GNU-compatible fallback expression.
+actual="$({
+    printf '%s\n' \
+        '00000000 l d .text discarded' \
+        '00000010 g F .text retained'
+} | ("$sed_bin" '/[[:<:]]d[[:>:]]/d' 2>/dev/null ||
+     "$sed_bin" '/\bd\b/d'))"
+
+expected='00000010 g F .text retained'
+
+if [ "$actual" != "$expected" ]; then
+    printf 'expected: %s\nactual:   %s\n' "$expected" "$actual" >&2
+    exit 1
+fi