PEG.jsによる逆ポ

test.pegjs

Statement
  = a:Expression _ b:Statement _ operator:Operator {return operator(a,b)}
  / Expression

Expression
  = a:Number _ b:Number _ operator:Operator {return operator(a,b)}
  / Number

Operator "operator"
  = "*" {return function(a,b){return a*b}}
  / "+" {return function(a,b){return a+b}}
  / "-" {return function(a,b){return a-b}}
  / "/" {return function(a,b){return a/b}}

Number
  = Double
   /Integer
Double
  = a:Integer "." b:Integer {return Number(text())};
Integer
  = [0-9]+ { return Number(text()); }

_ "whitespace"
  = [ \t\n\r]*

使い方.md

Try this here: http://pegjs.org/online