Expat is a stream-oriented XML 1.0 parser library, written in C, more precisely C99.

Such projects include the Apache HTTP Server, Mozilla, Perl, Python and PHP.

Software developer James Clark released version 1.0 in 1998 while serving as technical lead on the XML Working Group at the World Wide Web Consortium.

Versions exist for most[quantify] major[citation needed] operating-systems.

When Expat parses an XML document, it calls the registered handlers as it finds relevant tokens in the input stream.