fix regexp-replace*

collects/scheme/private/string.ss

@@ -328,9 +328,8 @@
                                              "expected a byte string result: ")
                                          v))
                  v))
-             (define rx:sub #rx#"^(?:[^&\\]*[\\][&\\])*[^&\\]*(?:&|[\\](?=[^&\\]|$))")
              (define need-replac? (and (not (procedure? replacement))
-                                       (regexp-match? rx:sub replacement)))
+                                       (regexp-match? #rx#"[\\&]" replacement)))
              (define (replac ms str)
                (if need-replac?
                    ((if (string? str) bytes->string/utf-8 values)

collects/tests/mzscheme/basic.ss

@@ -1071,6 +1071,8 @@
 (test "==1=2===3==4==" regexp-replace* "2*" "1234" (lambda (s) (string-append "=" s "=")))
 
 (test "x&b\\ab=cy&w\\aw=z" regexp-replace* #rx"a(.)" "xabcyawz" "\\&\\1\\\\&\\99=")
+(test "x&cy&z" regexp-replace* #rx"a(.)" "xabcyawz" "\\&")
+(test "x\\cy\\z" regexp-replace* #rx"a(.)" "xabcyawz" "\\\\")
 
 ;; Test weird port offsets:
 (define (test-weird-offset regexp-match regexp-match-positions)