From much searching of information on XACML it would appear one of the barriers to entry is the demand the specification places on policies being implemented in a verbose XML syntax. The Axiomatics Eclipse ALFA plugin is I understand not a free tool unless one engages the vendor.
I'd like to write my own open-source ALFA implementation that works cross-platform but want to make sure I'm ALFA compliant. I have the public ALFA spec so can write an Antlr4 g4 grammar but I'd rather not reinvent the wheel here.
Is there an ALFA g4 grammar in existence available to the open-source community?
The ALFA specification is now part of the OASIS XACML Technical Committee so you can definitely implement your own ALFA parser.
AFAIK The Axiomatics ALFA plugin for Eclipse is the only plugin currently available. I believe the vendor is working on a command-line version too. Their license terms make the plugin available for free for non-commercial use. If you are in academia, then you can use it.
As for XACML itself, you can easily create your own UI on top of the XACML syntax.