Naive solution

Author: michael-o

src/main/java/org/codehaus/plexus/util/xml/PrettyPrintXMLWriter.java

@@ -186,7 +186,7 @@ private void writeText(String text, boolean escapeXml) {
         finishTag();
 
         if (escapeXml) {
-            text = escapeXml(text);
+            text = escapeXmlText(text);
         }
 
         write(StringUtils.unifyLineSeparators(text, lineSeparator));
@@ -228,6 +228,8 @@ private static String escapeXml(String text) {
 
     private static final Pattern lowers = Pattern.compile("([\000-\037])");
 
+    private static final Pattern lowersText = Pattern.compile("([\000-\010\013-\014\016-\037])");
+
     private static String escapeXmlAttribute(String text) {
         text = escapeXml(text);
 
@@ -247,6 +249,19 @@ private static String escapeXmlAttribute(String text) {
         return b.toString();
     }
 
+    private static String escapeXmlText(String text) {
+        text = escapeXml(text);
+
+        Matcher m = lowersText.matcher(text);
+        StringBuffer b = new StringBuffer();
+        while (m.find()) {
+            m = m.appendReplacement(b, "&#" + Integer.toString(m.group(1).charAt(0)) + ";");
+        }
+        m.appendTail(b);
+
+        return b.toString();
+    }
+
     /** {@inheritDoc} */
     @Override
     public void addAttribute(String key, String value) {

src/main/java/org/codehaus/plexus/util/xml/pull/MXSerializer.java

@@ -943,19 +943,11 @@ protected void writeElementContent(String text, Writer out) throws IOException {
                         // out.write(';');
                         // pos = i + 1;
                     } else {
-                        throw new IllegalStateException(
-                                "character " + Integer.toString(ch) + " is not allowed in output" + getLocation());
-                        // in XML 1.1 legal are [#x1-#xD7FF]
-                        // if(ch > 0) {
-                        // if(i > pos) out.write(text.substring(pos, i));
-                        // out.write("&#");
-                        // out.write(Integer.toString(ch));
-                        // out.write(';');
-                        // pos = i + 1;
-                        // } else {
-                        // throw new IllegalStateException(
-                        // "character zero is not allowed in XML 1.1 output"+getLocation());
-                        // }
+                        if (i > pos) out.write(text.substring(pos, i));
+                        out.write("&#");
+                        out.write(Integer.toString(ch));
+                        out.write(';');
+                        pos = i + 1;
                     }
                 }
                 if (seenBracket) {

src/test/java/org/codehaus/plexus/util/xml/Xpp3DomWriterTest.java

@@ -82,7 +82,11 @@ private String createExpectedXML(boolean escape) {
         buf.append(LS);
         buf.append("  </el6>");
         buf.append(LS);
-        buf.append("  <el8>special-char-" + (char) 7 + "</el8>");
+        if (escape) {
+            buf.append("  <el8>special-char-&#7;</el8>");
+        } else {
+            buf.append("  <el8>special-char-" + (char) 7 + "</el8>");
+        }
         buf.append(LS);
         buf.append("</root>");