Handle the regexp matching the empty string specially, as POSIX

requires it to be "()", not "()".

This closes bug

[ 1005026 ] sre: (** 0 0 "foo") raises error

scsh/rx/re.scm

@@ -338,24 +338,40 @@
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;; Also, re-empty and re-trivial.
 
-(define-record re-string
-  chars			; String
-  (posix #f)		; Posix record
-  ((disclose self) (list "re-string" (re-string:chars self))))
+(define-record-type re-string :re-string
+  (really-make-re-string chars posix)
+  re-string?
+  (chars re-string:chars set-re-string:chars)
+  (posix re-string:posix set-re-string:posix))
+
+(define-record-discloser :re-string
+  (lambda (r)
+    (list 're-string
+	  (re-string:chars r))))
+
+;; Kludge: POSIX wants "()" for "the empty string".
+
+(define (make-re-string chars)
+  (if (string=? "" chars)
+      re-trivial
+      (really-make-re-string chars #f)))
 
 (define re-string make-re-string)	; For consistency w/other re makers.
 
 ;;; This is only used in code that (RE ...) macro produces for static regexps.
 (define (make-re-string/posix chars posix-str tvec)
-  (let ((re (make-re-string chars)))
-    (set-re-string:posix re (new-cre posix-str tvec))
-    re))
+  (if (string=? "" chars)
+      re-trivial
+      (really-make-re-string chars (new-cre posix-str tvec))))
+
+(define re-empty-string (really-make-re-string "" #f))
 
 ;;; Matches the empty string anywhere.
-(define re-trivial (make-re-string/posix "" "" '#()))
+(define re-trivial (make-re-dsm/posix re-empty-string
+				      1 0 "()" '#()))
 
 (define (re-trivial? re)
-  (and (re-string? re) (zero? (string-length (re-string:chars re)))))
+  (eq? re re-trivial))
 
 (define-record re-char-set
   cset			; A character set	(Macro expander abuses.)