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Improve regex documentation

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* doc/autoconf.texi (Running the Preprocessor)
(Limitations of Usual Tools):
Improve comments on limitations of regular expressions.
This commit is contained in:
Paul Eggert 2022-06-22 00:02:12 -05:00
parent 0c76267536
commit 256d85494e
1 changed files with 17 additions and 6 deletions
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doc/autoconf.texi
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@ -9702,8 +9702,10 @@ to run the @emph{preprocessor} and not the compiler?
@defmac AC_EGREP_HEADER (@var{pattern}, @var{header-file}, @
@var{action-if-found}, @ovar{action-if-not-found})
@acindex{EGREP_HEADER}
@var{pattern}, after being expanded as if in a double-quoted shell string,
is an extended regular expression.
If the output of running the preprocessor on the system header file
@var{header-file} matches the extended regular expression
@var{header-file} contains a line matching
@var{pattern}, execute shell commands @var{action-if-found}, otherwise
execute @var{action-if-not-found}.
@ -9714,10 +9716,12 @@ See below for some problems involving this macro.
@defmac AC_EGREP_CPP (@var{pattern}, @var{program}, @
@ovar{action-if-found}, @ovar{action-if-not-found})
@acindex{EGREP_CPP}
@var{pattern}, after being expanded as if in a double-quoted shell string,
is an extended regular expression.
@var{program} is the text of a C or C++ program, which is expanded as an
unquoted here-document (@pxref{Here-Documents}). If the
output of running the preprocessor on @var{program} matches the
extended regular expression @var{pattern}, execute shell commands
output of running the preprocessor on @var{program} contains a line
matching @var{pattern}, execute shell commands
@var{action-if-found}, otherwise execute @var{action-if-not-found}.
See below for some problems involving this macro.
@ -9750,6 +9754,8 @@ of things that do not change the meaning of the preprocessed program, it
is better to rely on @code{AC_PREPROC_IFELSE} than to resort to
@code{AC_EGREP_CPP} or @code{AC_EGREP_HEADER}.
For more information about what can appear in portable extended regular
expressions, @pxref{Problematic Expressions,,,grep, GNU Grep}.
@node Running the Compiler
@section Running the Compiler
@ -19360,6 +19366,9 @@ foo
|bar
@end example
For more information about what can appear in portable extended regular
expressions, @pxref{Problematic Expressions,,,grep, GNU Grep}.
@command{$EGREP} also suffers the limitations of @command{grep}
(@pxref{grep, , Limitations of Usual Tools}).
@ -19411,7 +19420,7 @@ Avoid this portability problem by avoiding the empty string.
@c ----------------------------
@prindex @command{expr}
Portable @command{expr} regular expressions should use @samp{\} to
escape only characters in the string @samp{$()*.0123456789[\^n@{@}}.
escape only characters in the string @samp{$()*.123456789[\^@{@}}.
For example, alternation, @samp{\|}, is common but Posix does not
require its support, so it should be avoided in portable scripts.
Similarly, @samp{\+} and @samp{\?} should be avoided.
@ -19615,13 +19624,15 @@ not use both @option{-E} and @option{-F}, since Posix does not allow
this combination.
Portable @command{grep} regular expressions should use @samp{\} only to
escape characters in the string @samp{$()*.0123456789[\^@{@}}. For example,
escape characters in the string @samp{$()*.123456789[\^@{@}}. For example,
alternation, @samp{\|}, is common but Posix does not require its
support in basic regular expressions, so it should be avoided in
portable scripts. Solaris and HP-UX @command{grep} do not support it.
Similarly, the following escape sequences should also be avoided:
@samp{\<}, @samp{\>}, @samp{\+}, @samp{\?}, @samp{\`}, @samp{\'},
@samp{\B}, @samp{\b}, @samp{\S}, @samp{\s}, @samp{\W}, and @samp{\w}.
For more information about what can appear in portable regular expressions,
@pxref{Problematic Expressions,,, grep, GNU Grep}.
Posix does not specify the behavior of @command{grep} on binary files.
An example where this matters is using BSD @command{grep} to
@ -19959,7 +19970,7 @@ $ @kbd{printf '\200\n' | LC_ALL=en_US.ISO8859-1 sed -n /./p | wc -l}
@end example
Portable @command{sed} regular expressions should use @samp{\} only to escape
characters in the string @samp{$()*.0123456789[\^n@{@}}. For example,
characters in the string @samp{$()*.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'}