not

not combinator applies parser without consuming any input and succeeds with null only if it fails, i.e. acts as the negative counterpart of lookahead. If parser succeeds, not fails with expected, which defaults to unexpected input.

A committed failure propagates instead of counting as a non-match, so wrap parser in backtrack to keep it speculative.

Usage

The example below parses the let keyword, rejecting identifiers that merely start with it, like letx.

const Parser = first(string('let'), not(letter(), 'keyword boundary'))

Success

run(Parser).with('let x')

{
  isOk: true,
  start: 0,
  end: 3,
  pos: 3,
  value: 'let'
}

Failure

run(Parser).with('letx')

{
  isOk: false,
  start: 3,
  end: 4,
  pos: 3,
  expected: 'keyword boundary'
}