Ordered choices

Choices are used to denote places where there is more than one possible syntax for the input to follow. For example the following grammar matches either the string "hello" or the string "world":

choice.peg

grammar Choice
  root  <-  "hello" / "world"

The important thing about choices in PEG grammars is that they’re ordered. This means that the options are tried in order and the first choice that leads to a successful parse is kept. If this choice results in a parsing error later on in the input, the parser does not backtrack and try any other choices, the parse simply fails.

The choice operator binds more loosely than the sequencing operator, for example the following grammar matches either the string "DouglasAdams" or the string "HitchhikersGuide":

binding.peg

grammar Binding
  root  <-  "Douglas" "Adams" / "Hitchhikers" "Guide"

If you just want the choice to be between the nodes right next to the / operator, you need to parenthesize them. This grammar matches "DouglasAdamsGuide" or "DouglasHitchhikersGuide":

binding.peg

grammar Binding
  root  <-  "Douglas" ("Adams" / "Hitchhikers") "Guide"