peggy

pegjs

start
  = operation

operation
  = WSS a:entity WSS b:operator WSS c:operation WSS { return `${a} ${b} ${c}`; }
  / WSS a:entity WSS b:operator2 WSS c:operation WSS { return `${a}${b}${c}`; }
  / WSS a:entity WSS c:operation WSS { return `${a}${c}`; }
  / WSS a:entity WSS { return a; }

entity "entity"
  = e:ident_part+ { return e.join(''); }
  / "{" WSS e:operation WSS "}" { return `{${e}}`; }
  / "(" WSS e:operation WSS ")" { return `(${e})`; }
  / b:built_in_operator e:ident_part+ { return `\\${e.join('')}`; }

built_in_operator "built_in_operator"
  = "\\"

ident_part
  = [A-Za-z0-9]

operator "operator"
  = [\+\-\*\/=]

operator2 "operator2"
  = [_^]

WSS "whitespaces"
  = [ \t\r\n]*

1+ \Omega_1^2      
 + a_a_1  + \frac{1+1}{i-1} /

             (a    +1      /3       )
/(4+5)*
9