Add a server for checking basic XML integrity

app/src/main/java/ch/nuth/zhaw/exbox/WellformedXmlServer.java

@@ -0,0 +1,30 @@
+package ch.nuth.zhaw.exbox;
+
+import java.util.regex.MatchResult;
+import java.util.regex.Pattern;
+
+public class WellformedXmlServer implements CommandExecutor {
+    Pattern tagPattern = Pattern.compile("<(/?)(\\w+)( \\w+=\"[^\"]*\")*>");
+
+    @Override
+    public String execute(String command) throws Exception {
+        return Boolean.toString(checkWellformed(command)) + "\n";
+    }
+
+    public boolean checkWellformed(String code) {
+        ListStack openTags = new ListStack();
+        Iterable<MatchResult> results = () -> tagPattern.matcher(code).results().iterator();
+
+        for (MatchResult result : results) {
+            if (result.group(1).length() == 0) {
+                openTags.push(result.group(2));
+            } else if (result.group(2).equals(openTags.peek())) {
+                openTags.pop();
+            } else {
+                return false;
+            }
+        }
+
+        return openTags.isEmpty();
+    }
+}