Good news: The REx Parser Generator was published under the Apache 2 license.
Nota bene:
As of today, REx is a parser generator supporting different parsing algorithms, with integrated lexer construction, for various host languages, implemented in C++. But the setup, focus, and implementation of the product have changed significantly over time, since the development of this line of parsing tools originally started in 1979.
"since the development of this line of parsing tools originally started in 1979" 
For a practical application see
or
Kosek, Jirka. 2017. „Improving Validation of Structured Text“. In XML London 2017 Conference Proceedings, 56–67. London, United Kingdom: XML London.