you can use replace() instead replaceAll() following works for me

String s = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\"?>";
String s2 = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\"?>";
s2 = s2.replace(s, "");
System.out.println(s2);

OP:

<blank>

EDIT:

how about following?

String s = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\"?>";
Scanner sc = new Scanner(new File("D:\\temp.txt"));
String s2 = sc.nextLine();
System.out.println("b4 "+s2);
s2 = s2.replaceAll(s, "");
System.out.println("aftr "+s2);

File Content :

<?xml version="1.0" encoding="UTF-8" standalone="no"?>

Very simply: use an XML library. That way it will actually be right instead of requiring detailed knowledge of bits of the XML spec.

public class RssParser {
int length;
    URL url;
URLConnection urlConn;
NodeList nodeList;
Document doc;
Node node;
Element firstEle;
NodeList titleList;
Element ele;
NodeList txtEleList;
String retVal, urlStrToParse, rootNodeName;

public RssParser(String urlStrToParse, String rootNodeName){
    this.urlStrToParse = urlStrToParse;
    this.rootNodeName = rootNodeName;

    url=null;
    urlConn=null;
    nodeList=null;
    doc=null;
    node=null;
    firstEle=null;
    titleList=null;
    ele=null;
    txtEleList=null;
    retVal=null;
            doc = null;
    try {
        url = new URL(this.urlStrToParse);
                    // dis is path of url which v'll parse
        urlConn = url.openConnection();

                    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();

        String s = isToString(urlConn.getInputStream());
        s = s.replace("&", "&");
        StringBuilder sb =
                            new StringBuilder
                                    ("<?xml version=\"1.0\" encoding=\"utf-8\"?>");
        sb.append("\n"+s);
        System.out.println("STR: \n"+sb.toString());
        s = sb.toString();

        doc = db.parse(urlConn.getInputStream());
        nodeList = doc.getElementsByTagName(this.rootNodeName); 
        //  dis is d first node which
        //  contains other inner element-nodes
        length =nodeList.getLength();
        firstEle=doc.getDocumentElement();
    }
    catch (ParserConfigurationException pce) {
        System.out.println("Could not Parse XML: " + pce.getMessage());
    }
    catch (SAXException se) {
        System.out.println("Could not Parse XML: " + se.getMessage());
    }
    catch (IOException ioe) {
        System.out.println("Invalid XML: " + ioe.getMessage());
    }
    catch(Exception e){
        System.out.println("Error: "+e.toString());
    }
}


public String isToString(InputStream in) throws IOException {
    StringBuffer out = new StringBuffer();
    byte[] b = new byte[512];
    for (int i; (i = in.read(b)) != -1;) {
        out.append(new String(b, 0, i));
    }
    return out.toString();
}

public String getVal(int i, String param){
    node =nodeList.item(i);
    if(node.getNodeType() == Node.ELEMENT_NODE)
    {
        System.out.println("Param: "+param);
        titleList = firstEle.getElementsByTagName(param);
        if(firstEle.hasAttribute("id"))
        System.out.println("hasAttrib----------------");
        else System.out.println("Has NOTNOT      NOT");
        System.out.println("titleList: "+titleList.toString());
    ele = (Element)titleList.item(i);
    System.out.println("ele: "+ele);
        txtEleList = ele.getChildNodes();
    retVal=(((Node)txtEleList.item(0)).getNodeValue()).toString();
    if (retVal == null)
        return null;
            System.out.println("retVal: "+retVal);
    }
return retVal;
}
}

All these answers so far only replace the characters themselves. But sometimes an XML document will have invalid XML entity sequences resulting in errors. For example, if you have  in your xml, a java xml parser will throw Illegal character entity: expansion character (code 0x2 at ....

Here is a simple java program that can replace those invalid entity sequences.

  public final Pattern XML_ENTITY_PATTERN = Pattern.compile("\\&\\#(?:x([0-9a-fA-F]+)|([0-9]+))\\;");

  /**
   * Remove problematic xml entities from the xml string so that you can parse it with java DOM / SAX libraries.
   */
  String getCleanedXml(String xmlString) {
    Matcher m = XML_ENTITY_PATTERN.matcher(xmlString);
    Set<String> replaceSet = new HashSet<>();
    while (m.find()) {
      String group = m.group(1);
      int val;
      if (group != null) {
        val = Integer.parseInt(group, 16);
        if (isInvalidXmlChar(val)) {
          replaceSet.add("&#x" + group + ";");
        }
      } else if ((group = m.group(2)) != null) {
        val = Integer.parseInt(group);
        if (isInvalidXmlChar(val)) {
          replaceSet.add("&#" + group + ";");
        }
      }
    }
    String cleanedXmlString = xmlString;
    for (String replacer : replaceSet) {
      cleanedXmlString = cleanedXmlString.replaceAll(replacer, "");
    }
    return cleanedXmlString;
  }

  private boolean isInvalidXmlChar(int val) {
    if (val == 0x9 || val == 0xA || val == 0xD ||
            val >= 0x20 && val <= 0xD7FF ||
            val >= 0x10000 && val <= 0x10FFFF) {
      return false;
    }
    return true;
  }

Please do not use regular expressions to escape special characters in XML.

Can you guarantee that this will work for all possible html input with all of HTML and XML quirks (very extensive specs!!!) ?

Just use one of many utilities out there to escape XML strings.

Apache Commons is quite popular - please see this example