[Grammar-Kit] non-closed block comments

Hi

What's the correct way to handle non-closed block comments in grammar-kit?  I copied this rule

BLOCK_COMMENT="/*" !([^]* "*/" [^]*) ("*/")?

from

./support/org/intellij/jflex/parser/_JFlexLexer.flex

and it seems to work, except that it makes closing "*/" optional. How to make parser fail on unclosed block comments?

Thanks