gistfile1.txt

#!/opt/rakudo-star-2016.10/bin/perl6
use v6;
#use Grammar::Tracer;
#use Grammar::Debugger;

# Question is about regex expr definition (see line 72/73):

#   When expr is defined as "regex expr  {:s {;}<mot1>* ", AST is 
#｢select decode(i_r_p_a,'O',0,nvl(dual.dummy,0) )  from dual ｣
# main => ｢select decode(i_r_p_a,'O',0,nvl(dual.dummy,0) )  ｣
#  text => ｢select ｣
#  decode => ｢decode(i_r_p_a,'O',0,nvl(dual.dummy,0) )  ｣
#   decodeinstr => ｢decode｣
#   paraIn => ｢(｣
#   val => ｢i_r_p_a｣
#    expr => ｢i_r_p_a｣
#     mot1 => ｢i_r_p_a｣
#   whenthen => ｢,'O',0｣
#    comma => ｢,｣
#    when => ｢'O'｣
#     expr => ｢'O'｣
#      mot1 => ｢'｣
#      mot1 => ｢O｣
#      mot1 => ｢'｣
#    comma => ｢,｣
#    then => ｢0｣
#     expr => ｢0｣
#      mot1 => ｢0｣
#   comma => ｢,｣
#   else => ｢nvl(dual.dummy,0) ｣
#    expr => ｢nvl(dual.dummy,0) ｣
#     mot1 => ｢nvl｣
#     paraIn => ｢(｣
#     mot => ｢dual｣
#     mot => ｢.｣
#     mot => ｢dummy｣
#     mot => ｢,｣
#     mot => ｢0｣
#     paraOut => ｢) ｣
#   paraOut => ｢)  ｣
# textfin => ｢from dual ｣

#   When expr is defined as "regex expr  {:s <mot1>* ", AST is 
#｢select decode(i_r_p_a,'O',0,nvl(dual.dummy,0) )  from dual ｣
# textfin => ｢select decode(i_r_p_a,'O',0,nvl(dual.dummy,0) )  from dual ｣

# I don't understand why I have to add {;} to get the "correct" AST. Any idea ?
 
grammar DECODE {
 
    rule  TOPDECODE     { [<main>]* <textfin>? }
      rule  main     { <text>? <decode> }
        rule  decode {:i <decodeinstr> <paraIn> <val> <whenthen>+ [<comma> <else>] <paraOut>
                      |  <decodeinstr> <paraIn> <val> <whenthen>+ <paraOut> }
          regex decodeinstr {:i 'decode'\s* }
          regex val    {:s <decode>
                           |<expr> }
          regex  whenthen {:i <comma> <when> <comma> <then> }
            regex when    {:s <decode>
                             | <expr> }
            regex then    {:s <decode>
                             |<expr> }
            regex else    {:s <decode>
                             |<expr> }
        regex text    {:s .+? }
      regex textfin {:s .+ }
      regex comma   {\s*','\s* }
      regex paraIn  {\s*'('\s*}
      regex paraOut {\s*')'\s*}
      token ws { <!ww> }
     ########  EXPR DEFINITION CHOOSE TO COMMENT LINE 72 or 73
      #regex expr  {:s {;}<mot1>*
      regex expr  {:s <mot1>*
                      | [<mot1><paraIn><mot>*<paraOut>]*
                      | [<paraIn><mot>*<paraOut>]*
                  }
      rule mot  { <-[()]>+?  }
      rule mot1  { <-[(),]>+?  }

}
 
 
my $sqlStmt="select decode(i_r_p_a,'O',0,nvl(dual.dummy,0) )  from dual ";
say DECODE.parse($sqlStmt, rule => 'TOPDECODE');