Optional nodes

To make a node optional, follow it immediately with a ?, with no space between the node and the ? symbol.

Here’s a grammar that matches "future" or the empty string:

future.peg
grammar Future
  root  <-  "future"?

Here’s a grammar that matches any digit or the empty string:

digit.peg
grammar Digit
  root  <-  [0-9]?

If the optional node is not found in the input, an empty syntax node is returned. Remember an optional node either matches the node or nothing at all; other input will cause an error.

require('./future').parse('future')
   == { text: 'future', offset: 0, elements: [] }

require('./future').parse('')
   == { text: '', offset: 0, elements: [] }

require('./future').parse('perfect')
Error: Line 1: expected "future"
perfect
^