joni interprets [\w-#] as [ !"#0-9A-Z_a-z]

Joni interprets [\w-#] as [ !"#0-9A-Z_a-z] in both default syntax and Java syntax. Java Pattern interprets it as [-0-9A-Z_a-z].

An addition question: in general, is it considered a bug if interpretation doesn't match Java pattern when syntax is set to Java?

Yes, if the Java dialect does not match Java pattern behavior, we should fix that.