forgotten-attachment: fix empty regex expression

The original regex was of the form "abc(|def)" to check for both "abc"
and "abcdef".  Unfortunately, the regex libraries on BSDs/MacOS don't
like this use of an empty sub-expression.

Expanding the regex to: "(abc|abcdef)" fixes the problem.

Fixes: neomutt/homebrew-neomutt#15

doc/manual.xml.head

@@ -10142,7 +10142,7 @@ set pgp_encrypt_self = "no"
           <row>
             <entry><literal>attach_keyword</literal></entry>
             <entry>regular expression</entry>
-            <entry><literal>\\&lt;attach(|ed|ments?)\\&gt;</literal></entry>
+            <entry><literal>\\&lt;(attach|attached|attachments?)\\&gt;</literal></entry>
           </row>
           <row>
             <entry><literal>abort_noattach</literal></entry>

doc/muttrc.forgotten-attachment

@@ -16,7 +16,7 @@ set abort_noattach = no
 # Search for the following regular expression in the body of the email
 
 # English: attach, attached, attachment, attachments
-set attach_keyword = "\\<attach(|ed|ments?)\\>"
+set attach_keyword = "\\<(attach|attached|attachments?)\\>"
 
 # Nederlands:
 # set attach_keyword = "\\<(bijvoegen|bijgevoegd|bijlage|bijlagen)\\>"

init.c

@@ -1704,6 +1704,7 @@ static void mutt_restore_default (struct option_t *p)
 
 	if (p->init)
 	{
+	  int retval;
 	  char *s = (char *) p->init;
 
 	  pp->rx = safe_calloc (1, sizeof (regex_t));
@@ -1715,10 +1716,15 @@ static void mutt_restore_default (struct option_t *p)
 	    s++;
 	    pp->not = 1;
 	  }
-	  if (REGCOMP (pp->rx, s, flags) != 0)
+	  retval = REGCOMP (pp->rx, s, flags);
+	  if (retval != 0)
 	  {
+	    char msgbuf[STRING];
+	    regerror (retval, pp->rx, msgbuf, sizeof (msgbuf));
 	    fprintf (stderr, _("mutt_restore_default(%s): error in regexp: %s\n"),
 		     p->option, pp->pattern);
+	    fprintf (stderr, "%s\n", msgbuf);
+	    mutt_sleep (0);
 	    FREE (&pp->pattern);
 	    FREE (&pp->rx);
 	  }

init.h

@@ -267,7 +267,7 @@ struct option_t MuttVars[] = {
   ** .pp
   ** For an explanation of ``soft-fill'', see the $$index_format documentation.
   */
-  { "attach_keyword",  DT_RX,  R_NONE, UL &AttachKeyword, UL "\\<attach(|ed|ments?)\\>" },
+  { "attach_keyword",  DT_RX,  R_NONE, UL &AttachKeyword, UL "\\<(attach|attached|attachments?)\\>" },
   /*
   ** .pp
   ** If $abort_noattach is not set to no, then the body of the message

send.c

@@ -1902,7 +1902,7 @@ ci_send_message (int flags,		/* send mode */
     /* if the abort is automatic, print an error message */
     if (quadoption (OPT_ATTACH) == MUTT_YES)
     {
-      mutt_error _("Message contains text matching \"attach_keyword\". Not sending.");
+      mutt_error _("Message contains text matching \"$attach_keyword\". Not sending.");
     }
     goto main_loop;
   }