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Document s/a/\n/ etc

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Problem reported by Bruno Haible in:
https://lists.gnu.org/r/autoconf-patches/2025-06/msg00001.html
* doc/autoconf.texi (Limitations of Usual Tools):
Also mention escapes in replacement strings.
While we’re at it, update -e and -f concatenation doc.
This commit is contained in:
Paul Eggert 2025-06-25 11:28:24 -07:00
parent 416b8f50c9
commit cbfc5f6cd1
1 changed files with 28 additions and 20 deletions
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@ -19790,14 +19790,6 @@ $ @kbd{printf '\200\n' | LC_ALL=en_US.ISO8859-1 sed -n /./p | wc -l}
1
@end example
Portable @command{sed} regular expressions should use @samp{\} only to escape
characters in the string @samp{$()*.123456789[\^n@{@}}. For example,
alternation, @samp{\|}, is common but POSIX does not require its
support, so it should be avoided in portable scripts. Solaris
@command{sed} does not support alternation; e.g., @samp{sed '/a\|b/d'}
deletes only lines that contain the literal string @samp{a|b}.
Similarly, @samp{\+} and @samp{\?} should be avoided.
Anchors (@samp{^} and @samp{$}) inside groups are not portable.
Nested parentheses in patterns (e.g., @samp{\(\(a*\)b*)\)}) are
@ -19817,6 +19809,27 @@ $ @kbd{echo '1*23*4' | /usr/xpg4/bin/sed 's/\(.\)*/x/g'}
x
@end example
Portable @command{sed} regular expressions should use @samp{\} only to escape
characters in the string @samp{$()*.123456789[\^n@{@}}. For example,
alternation, @samp{\|}, is common but POSIX does not require its
support, so it should be avoided in portable scripts. Solaris
@command{sed} does not support alternation; e.g., @samp{sed '/a\|b/d'}
deletes only lines that contain the literal string @samp{a|b}.
Similarly, @samp{\+} and @samp{\?} should be avoided.
Portable @command{sed} replacement strings should use @samp{\} only to
escape the delimiter character, newline, and characters in the string
@samp{&123456789\}. For example, although in GNU @command{sed}
the command @samp{s/AB/\n/} replaces the first @samp{AB} with a newline,
in OpenBSD and Solaris @command{sed} it acts like @samp{s/AB/n/}.
To be portable, escape a newline instead:
@example
# This is portable; "sed 's/AB/\n/'" is not.
sed 's/AB/\
/'
@end example
The @option{-e} option is mostly portable.
However, its argument cannot be empty, as this fails on AIX 7.3.
Some people prefer to use @samp{-e}:
@ -19849,21 +19862,16 @@ but POSIX says that this use of a semicolon has undefined effect if
should use semicolon only with simple scripts that do not use these
verbs.
POSIX up to the 2008 revision requires the argument of the @option{-e}
option to be a syntactically complete script. GNU @command{sed} allows
to pass multiple script fragments, each as argument of a separate
@option{-e} option, that are then combined, with newlines between the
fragments, and a future POSIX revision may allow this as well. This
approach is not portable with script fragments ending in backslash; for
POSIX requires each @option{-e} and @option{-f} option to specify a
syntactically complete script. Although GNU @command{sed} also allows
@option{-e} and @option{-f} options to specify script fragments
that it assembles into a full script, this is not portable. For
example, the @command{sed} programs on Solaris 10, HP-UX 11, and AIX
don't allow splitting in this case:
do not allow script fragments:
@example
$ @kbd{echo a | sed -n -e 'i\}
@kbd{0'}
0
$ @kbd{echo a | sed -n -e 'i\' -e 0}
Unrecognized command: 0
$ @kbd{sed -e 'i\' -e ouch}
Unrecognized command: ouch
@end example
@noindent