fix raco pkg show --rx on bad patterns

Fix error reporting so it's not an internal error. Also,
report errors before printing a scope, and update the docs
for earlier changes.

Closes #1251

pkgs/racket-doc/pkg/scribblings/lib.scrbl

@@ -399,19 +399,32 @@ The @racket[name] parameter is the name of the new package.
 
 
 @defproc[(pkg-show [indent string?]
+                   [pkgs-or-patterns (or/c #f (listof string?))]
+                   [#:prefix-line prefix-line (or/c #f string?) #f]
                    [#:auto? auto? boolean? #f]
+                   [#:rx? rx? boolean? #f]
                    [#:long? long? boolean? #f]
+                   [#:full-checksum? full-checksum? boolean? #f]
                    [#:directory show-dir? boolean? #f])
          void?]{
 
 Implements @racket[pkg-show-command] for a single package scope,
-printing to the current output port. See also
+printing to the current output port. If @racket[prefix-line]s is not
+@racket[#f], it is printed before the output. See also
 @racket[installed-pkg-names] and @racket[installed-pkg-table].
 
+If @racket[pkgs-or-patterns] is @racket[#f], then information is shown
+for all installed packages, otherwise only matching packages are shown.
+In @racket[rx?] is true, then elements of @racket[pkgs-or-patterns]
+are treated as regular expression patterns, otherwise they are treated
+as package names.
+
 The package lock must be held to allow reads; see
 @racket[with-pkg-lock/read-only].
 
-@history[#:changed "6.1.1.5" @elem{Added the @racket[#:long?] argument.}]}
+@history[#:changed "6.1.1.5" @elem{Added the @racket[#:long?] argument.}
+         #:changed "6.1.1.6" @elem{Added the @racket[#:full-checksum?] and @racket[#:rx?] arguments.}
+         #:changed "6.5.0.1" @elem{Added the @racket[#:prefix-line] argument.}]}
 
 
 @defproc[(pkg-migrate      [from-version string?]

racket/collects/pkg/lib.rkt

@@ -134,11 +134,12 @@
         (or/c #f 'skip (listof (or/c path-string? (non-empty-listof path-string?)))))]
   [pkg-show
    (->* (string? (or/c #f (listof string?)))
-        (#:directory? boolean?
+        (#:prefix-line (or/c #f string?)
-                      #:long? boolean?
+                       #:directory? boolean?
-                      #:auto? boolean?
+                       #:long? boolean?
-                      #:full-checksum? boolean?
+                       #:auto? boolean?
-                      #:rx? boolean?)
+                       #:full-checksum? boolean?
+                       #:rx? boolean?)
         void?)]
   [pkg-install
    (->* ((listof pkg-desc?))

racket/collects/pkg/main.rkt

@@ -395,7 +395,7 @@
              [#:bool all ("-a") "Show auto-installed packages, too"]
              [#:bool long ("-l") "Show full column content"]
              [#:bool full-checksum () "Show the full checksum"]
-             [#:bool rx () "Treat <pkgs> as regular expressions"]
+             [#:bool rx () "Treat <pkg>s as regular expressions"]
              [#:bool dir ("-d") "Show the directory where the package is installed"]
              #:once-any
              scope-flags ...
@@ -420,17 +420,19 @@
                                               (simple-form-path d)))))
                                      '(user)))])
                (when (or (equal? mode only-mode) (not only-mode))
-                 (unless only-mode
+                 (define prefix-line
-                   (printf "~a\n" (case mode
+                   (and (not only-mode)
-                                    [(installation) "Installation-wide:"]
+                        (case mode
-                                    [(user) (format "User-specific for installation ~s:"
+                          [(installation) "Installation-wide:"]
-                                                    (or version (get-installation-name)))]
+                          [(user) (format "User-specific for installation ~s:"
-                                    [else (format "~a:" mode)])))
+                                          (or version (get-installation-name)))]
+                          [else (format "~a:" mode)])))
                  (parameterize ([current-pkg-scope mode]
                                 [current-pkg-error (pkg-error 'show)]
                                 [current-pkg-scope-version (or version (get-installation-name))])
                    (with-pkg-lock/read-only
                     (pkg-show (if only-mode "" " ") pkgs*
+                              #:prefix-line prefix-line
                               #:auto? all
                               #:long? long
                               #:rx? rx

racket/collects/pkg/private/show.rkt

@@ -6,26 +6,34 @@
          racket/path
          "../path.rkt"
          "dirs.rkt"
-         "pkg-db.rkt")
+         "pkg-db.rkt"
+         "print.rkt")
 
 (provide pkg-show)
 
 (define (pkg-show indent only-pkgs
+                  #:prefix-line [prefix-line #f]
                   #:directory? [dir? #f]
                   #:auto? [show-auto? #f]
                   #:full-checksum? [full-checksum #f]
                   #:long? [long? #t]
-                  #:rx? [rx #f]
+                  #:rx? [rx? #f]
                   #:name [name 'pkg-show])
-  (when (and rx (not only-pkgs))
+  (when rx?
-    (raise-user-error name "regular expression flag does not make sense without package names"))
+    (when (not only-pkgs)
+      (pkg-error "regular-expression mode requires at least one pattern"))
+    (for ([str (in-list only-pkgs)])
+      (regexp str (lambda (s)
+                    (pkg-error (~a "bad regular-expression pattern;\n"
+                                   " " s "\n"
+                                   "  in: " str))))))
   (define db (read-pkg-db))
   (define pkgs (sort (hash-keys db) string-ci<=?))
   (define auto-shown? #f)
   (define to-show
     (for/list ([pkg (in-list pkgs)]
                #:unless (and only-pkgs
-                             (not (memf (λ (v) (if rx (regexp-match? v pkg) (equal? v pkg)))
+                             (not (memf (λ (v) (if rx? (regexp-match? v pkg) (equal? v pkg)))
                                         only-pkgs)))
                #:when (or show-auto? only-pkgs
                           (not (pkg-info-auto? (hash-ref db pkg)))))
@@ -59,6 +67,8 @@
                        (~s p)
                        (~a p))))
            empty))))
+  (when prefix-line
+    (printf "~a\n" prefix-line))
     (if (null? to-show)
         (printf " [none]\n")
         (let* ([col-headers (list* (format "~aPackage~a"