Document another portability pitfall of sed

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With GNU sed:

$ echo foo | sed -e 's/f.*/line1\nline2/'
line1
line2

Even in strict POSIX mode:

$ echo foo | POSIXLY_CORRECT=1 sed --posix -e 's/f.*/line1\nline2/'
line1
line2

Whereas with OpenBSD sed and Solaris 11.4 sed:

$ echo foo | sed -e 's/f.*/line1\nline2/'
line1nline2

Here's a proposed patch to update the Autoconf documentation.

Note: I think POSIX [1] mandates the behaviour of OpenBSD and Solaris sed.
But that is not immediately relevant for the Autoconf manual: The portability
problem exists regardless of how we might interpret POSIX.

Bruno

[1] https://pubs.opengroup.org/onlinepubs/9799919799/utilities/sed.html

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From 8b043c4d205620687b7635d40a599c9bc16c6e2f Mon Sep 17 00:00:00 2001
From: Bruno Haible <[email protected]>
Date: Mon, 23 Jun 2025 23:29:29 +0200
Subject: [PATCH] doc: Document another portability pitfall of sed.

* doc/autoconf.texi (Limitations of Usual Tools): Mention escape sequences
in the replacement text of sed 's' commands.
---
 doc/autoconf.texi | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 12734446..70217a7f 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -19743,6 +19743,10 @@ Avoid empty patterns within parentheses (i.e., @samp{\(\)}).  POSIX does
 not require support for empty patterns, and Unicos 9 @command{sed} rejects
 them.
 
+In the replacement text of an @samp{s}, it is not portable to use
+escape sequences like @samp{\n}.  While GNU @command{sed} interprets
+them, the @command{sed} commands of OpenBSD and Solaris don't.
+
 Unicos 9 @command{sed} loops endlessly on patterns like @samp{.*\n.*}.
 
 Sed scripts should not use branch labels longer than 7 characters and
-- 
2.43.0


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