add xml-attribute-encode function

pkgs/racket-test/tests/xml/test.rkt

@@ -540,6 +540,10 @@ END
       
       (test-suite
        "xml->xexpr"
+
+       (test-equal? "xml-attribute-encode"
+                    (xml-attribute-encode "ab\"cd?e;;<i> %&quot;f")
+                    "ab&quot;cd?e;;&lt;i&gt; %&amp;quot;f")
        (test-xml->xexpr    
         "<doc><bold>hi</bold> there!</doc>"
         '(doc () (bold () "hi") " there!"))

racket/collects/xml/private/writer.rkt

@@ -166,7 +166,7 @@
     [(#\") """]
     [else c]))
 
-;; escape : String -> String
+;; escape : String Regexp -> String
 (define (escape x table)
   (regexp-replace* table x replace-escaped))
 

racket/collects/xml/private/xexpr.rkt

@@ -101,6 +101,7 @@
  [xml->xexpr (content/c . -> . xexpr/c)]
  [xexpr->xml (xexpr/c . -> . content/c)]
  [xexpr-drop-empty-attributes (parameter/c boolean?)]
+ [xml-attribute-encode (string? . -> . string?)]
  [write-xexpr (->* (xexpr/c)
                    (output-port?
                     #:insert-newlines? any/c)
@@ -168,3 +169,9 @@
       [(p-i? x)
        (write-xml-p-i x 0 void out)]))
   (void))
+
+;; given a string, encode it in the style required for attributes. Specifically,
+;; double-quote must be encoded as well as <, >, and &, because the double-quote
+;; would otherwise end the attribute.
+(define (xml-attribute-encode str)
+  (escape str escape-attribute-table))