How to reference attribute from .bnf parser in JFlex?

I'm using a .bnf parser to detect specific expressions and I'm using JFlex to detect the different sections of these expressions. My issue is, some of these expressions may contain nested expressions and I dont know how to handle that.

I've tried to include the .bnf parser in my JFlex by using %include, then referencing the expression in the relative macro using PARAMETERS = ("'"[:jletter:] [:jletterdigit:]*"'") | expression. This fails as JFlex reports the .bnf to be malformed.

Snippet of JFlex:

%{
  public Lexer() {
    this((java.io.Reader)null);
  }
%}

%public
%class Lexer
%implements FlexLexer
%function advance
%type IElementType
%include filename.bnf
%unicode

PARAMETERS= ("'"[:jletter:] [:jletterdigit:]*"'") | <a new expression element>

%%

<YYINITIAL> {PARAMETERS}   {return BAD_CHARACTER;} some random return

Snippet of .bnf parser:

{
//list of classes used.
}
expression ::= (<expression definition>)

Any input would be greatly appreciated. Thanks.

I've found the solution to my issue. In further depth, the problem was in both my grammar file and my flex file. To solve the issue, I recursively called the expression in the grammar file like so: expression = (start value expression? end)

With the JFlex, I declared numerous states until I found a way to chain together and endless amount of expressions. Looks a little like this:

%state = WAITING_EXPRESSION

<WAITING_NEXT> "<something which indicates start of nested expression>"   { yybegin(WAITING_EXPRESSION); return EXPRESSION_START; }