I tried to use GraalVM JavaScript on OpenJDK 12 and I got the following error message when evaluating a JavaScript file with a regular expression in it:

cannot access class com.oracle.truffle.regex.RegexLanguage (in module com.oracle.truffle.regex) because module com.oracle.truffle.regex does not export com.oracle.truffle.regex to module org.graalvm.truffle

I suppose TRegEx is missing an exports statement in its module-info, because when I added --add-exports com.oracle.truffle.regex/com.oracle.truffle.regex=org.graalvm.truffle to my java command, everything worked fine.