vendor/pest_derive/tests/opt.pest - toolchain/rustc - Git at Google

 expr = {
   SOI ~
   #prefix=(STAR)? ~ #suffix=DOT?
   ~ EOI
 }

 STAR={"*"}
 DOT={"."}
	expr = {
	SOI ~
	#prefix=(STAR)? ~ #suffix=DOT?
	~ EOI
	}

	STAR={"*"}
	DOT={"."}