as3判断XML是否合法

xml version="1.0" encoding="utf-8"?>
cc

<root><a/>ccroot>

ASDASDASDASD

ASDASD<root><a/>ccroot>

asdasd<root>

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var tmpByte:ByteArray = event.currentTarget.data;
var tmpText:String = "";
var tmpXml:XML;
try
{
    tmpXml = new XML(tmpByte.readUTFBytes(tmpByte.length));
    
    tmpText = tmpXml.toString();
    
    if(!(tmpText.substr(0,1) == "<" && tmpText.substr(tmpText.length - 1, 1) == ">"))
    {
        tmpXml = null;
    }
    else
    {
        tmpText = tmpXml.toString();
    }
} 
catch (error:Object) { tmpXml = null; }

if (tmpXml == null)
{
    try
    {
        tmpByte.uncompress(CompressionAlgorithm.ZLIB);
        
        tmpXml = new XML(tmpByte.readUTFBytes(tmpByte.length));
        
        tmpText = tmpXml.toString();
        
        if(!(tmpText.substr(0,1) == "<" && tmpText.substr(tmpText.length - 1, 1) == ">"))
        {
            tmpXml = null;
        }
        else
        {
            tmpText = tmpXml.toString();
        }
    }
    catch (error:Object) { tmpXml = null; }
    
    if (tmpXml == null)
    {
        tmpText = tmpByte.readUTFBytes(tmpByte.length);
    }
}
if (tmpXml == null)
{
    trace("此字符不是XML");
}
else
{
    trace("此数据是XML")
}