Expat is a non-validating XML parser written by James Clark and released under an Open Source license. This implies I only compiled it and built this installer package, but I did not not develop the software.
Most users won't ever use expat directly (although the included xmlwf application can be handy to check the wellformedness of an XML document, if xmllint is not available), but it is a prerequisite for some other software, for instance some XML-related Perl modules or the Sablotron XSL-T processor.
This installer will install expat compiled from the original, unmodified sources.
System requirements
The package was created on Mac OS X 10.3.6 and was tested on Mac OS X 10.2 and 10.3 successfully. I can't tell whether it will work on 10.1 or not.