Improve word detection for ESC . and ESC _ (emacs) and _ (vi)

These line editor commands insert the last word of the lsat command
in the history. About that, @davies42 reports:

> afaict M-. currently inserts the last word of the last command as
> defined only by splitting on spaces
>
> it would be nice to have at least a little of the improvement
> readline provides for this action, e.g. that it should also split
> on redirect instructions -- if i do
>
>     ... >file
>     cat <M-.>
>
> then in bash the second command is 'cat file', but in ksh, it's
> 'cat >file', which is actively harmful, since it wipes 'file'

src/cmd/ksh93/edit/{hexpand,history}.c:
- Move the static is_wordboundary function from hexpand.c to an
  extern hist_iswordbndry in history.c; we need it for hist_word.
- hist_word():
  - Instead of isspace, use hist_iswordbndry for word boundaries.
    This fixes the reported problem.
  - While we're here, fix another issue: add code to correctly
    parse $'...' quotes, so that a qutoed word like $'one \' two'
    is parsed correctly. (re: 7439e3d)

Resolves: #802

Author: McDutchie

NEWS

@@ -7,6 +7,13 @@ Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
 - Fixed crash upon resizing the terminal window after 'stty -echo', including
   when invoking ksh as a child shell in an editor such as emacs (M-x shell).
 
+- The ESC . and ESC _ commands in emacs/gmacs and the _ command in vi, which
+  insert the last word of the last command, now:
+  - recognise not only spaces but also the following shell grammar
+    characters as word boundaries: |&;()`<>
+  - correctly recognise a word that is quoted with the $'...' syntax,
+    even when it includes a backslash-escaped single quote.
+
 2025-03-24:
 
 - Fixed a crash that occurred in specific circumstances when processing a

src/cmd/ksh93/edit/hexpand.c

@@ -126,16 +126,6 @@ static char *parse_subst(const char *s, struct subst *sb)
 	return cp;
 }
 
-/*
- * return true if c is a word boundary character, i.e. the
- * character following c is considered to start a new word
- */
-
-static int is_wordboundary(char c)
-{
-	return isspace(c) || strchr("|&;()`<>",c);
-}
-
 /*
  * assign history expansion characters to an array of 3
  */
@@ -212,7 +202,7 @@ int hist_expand(const char *ln, char **xp)
 
 		if(hc[2] && *cp == hc[2])
 		{
-			if(cp == ln || is_wordboundary(cp[-1]))
+			if(cp == ln || hist_iswordbndry(cp[-1]))
 			{
 				/* word begins with history comment character; skip rest of line */
 				sfputr(sh.stk,cp,0);

src/cmd/ksh93/edit/history.c

@@ -998,13 +998,23 @@ int hist_copy(char *s1,int size,int command,int line)
 	return count;
 }
 
+/*
+ * return true if c is a word boundary character, i.e. the
+ * character following c is considered to start a new word
+ */
+
+int hist_iswordbndry(char c)
+{
+	return isspace(c) || strchr("|&;()`<>",c);
+}
+
 /*
  * return word number <word> from command number <command>
  */
 char *hist_word(char *string,int size,int word)
 {
 	int c;
-	int is_space;
+	int is_boundary;
 	int quoted;
 	char *s1 = string;
 	unsigned char *cp = (unsigned char*)s1;
@@ -1015,15 +1025,15 @@ char *hist_word(char *string,int size,int word)
 	hist_copy(string,size,(int)hp->histind-1,-1);
 	for(quoted=0;c = *cp;cp++)
 	{
-		is_space = isspace(c) && !quoted;
-		if(is_space && flag)
+		is_boundary = !quoted && hist_iswordbndry(c);
+		if(is_boundary && flag)
 		{
 			*cp = 0;
 			if(--word==0)
 				break;
 			flag = 0;
 		}
-		else if(is_space==0 && flag==0)
+		else if(is_boundary==0 && flag==0)
 		{
 			s1 = (char*)cp;
 			flag++;
@@ -1038,6 +1048,11 @@ char *hist_word(char *string,int size,int word)
 			for(cp++;*cp && (*cp != c || quoted);cp++)
 				quoted = *cp=='\\' ? !quoted : 0;
 		}
+		else if (c=='$' && cp[1]=='\'' && !quoted)
+		{
+			for(cp+=2; *cp && (*cp != '\'' || quoted); cp++)
+				quoted = *cp=='\\' ? !quoted : 0;
+		}
 		quoted = *cp=='\\' ? !quoted : 0;
 	}
 	*cp = 0;

src/cmd/ksh93/include/history.h

@@ -83,6 +83,7 @@ extern void 		hist_list(History_t*,Sfio_t*, off_t, int, char*);
 extern int		hist_match(History_t*,off_t, char*, int*);
 extern off_t		hist_tell(History_t*,int);
 extern off_t		hist_seek(History_t*,int);
+extern int		hist_iswordbndry(char);
 extern char 		*hist_word(char*, int, int);
 #if !_BLD_ksh || SHOPT_ESH
     extern Histloc_t	hist_locate(History_t*,int, int, int);