I've setup a small sample project which gets build by maven.
When BYACC/J generates the parser it puts the full path name of the grammar source file in java comments within the generated Parser.java file.
When compiling on Windows and any directory in the pathname of the grammar file starts with a lowercase U, like in the sample project, BYACC/J prints something like "C:\...\utils\src\main\java\grammar.y" within the java comment line.
When it comes to code compilation the java compiler complains about an invalid unicode character (\utils) in the path name.
I would recommend to replace all backslashes by slahes within the path names.
I appended a sample Eclipse-Java-Project that can be build by maven 2.
Sample Maven Project