example.c

# YYSTYPE is char *

Attribute   = '[' n:Id                             
            ( '='  v:Value                         { printf("%s %s\n", n, v) }       
            | '~=' v:Value                         
            | '^=' v:Value                         
            | '$=' v:Value                         
            | '*=' v:Value                        
            )? ']'                                 

Value      = Id | String

String     = '"' < (!'"' .)* > '"'             { $$ = yytext }
           | '\'' < (!'\'' .)* > '\''          { $$ = yytext }

Id         = < ([a-zA-Z0-9_] | '-')+ >         { $$ = yytext }