grep: warn about stray backslashes

This papers over a problem reported by Benno Schulenberg and
Tomasz Dziendzielski <https://bugs.gnu.org/39678> involving
regular expressions like \a that have unspecified behavior.
* src/dfasearch.c (dfawarn): Just output a warning.
Don’t exit, as DFA_CONFUSING_BRACKETS_ERROR now
does that for us, and we need the ability to warn
without exiting to diagnose \a etc.
(GEAcompile): Use new dfa options DFA_CONFUSING_BRACKETS_ERROR and
DFA_STRAY_BACKSLASH_WARN.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/dfasearch.c b/src/dfasearch.c
index 8f48296a..7547a8a3 100644
--- a/src/dfasearch.c
+++ b/src/dfasearch.c
@@ -53,10 +53,10 @@ dfaerror (char const *mesg)
   die (EXIT_TROUBLE, 0, "%s", mesg);
 }
 
-_Noreturn void
+void
 dfawarn (char const *mesg)
 {
-  dfaerror (mesg);
+  error (0, 0, _("warning: %s"), mesg);
 }
 
 /* If the DFA turns out to have some set of fixed strings one of
@@ -196,7 +196,8 @@ GEAcompile (char *pattern, idx_t size, reg_syntax_t syntax_bits,
 
   if (match_icase)
     syntax_bits |= RE_ICASE;
-  int dfaopts = eolbyte ? 0 : DFA_EOL_NUL;
+  int dfaopts = (DFA_CONFUSING_BRACKETS_ERROR | DFA_STRAY_BACKSLASH_WARN
+                 | (eolbyte ? 0 : DFA_EOL_NUL));
   dfasyntax (dc->dfa, &localeinfo, syntax_bits, dfaopts);
   bool bs_safe = !localeinfo.multibyte | localeinfo.using_utf8;