This is James Clark's Expat XML parser library in C. It is a stream oriented parser that requires setting handlers to deal with the structure that the parser discovers in the document.
libexpat.so.0 is still in use by Avast Anti-Virus. I use both Avast and Windows. So I run virus scans in Linux before I boot into windows. And updating the Expat removes Avast from my system due to the dependancy on libexpat.so.0