Designate the first lookahead element of a rule. The first node matching the lookahead element or to the right of it will be the locus where the rule matcher continues matching, after finishing with the current rule.
Unlike YACC, BISON, or other grammar frameworks that use a lookahead or "LR" grammar, NLP++ rules require that lookahead constraints, if any, be specified explicitly.
look is an alternate form for lookahead.
A reduce action such as singler
or noop should be used to ensure
that the lookahead node and nodes to its right are not included in the
current rule reduction.
# The rule below reduces a determiner, quantifier, adjective pattern
to a noun phrase as long as the subsequent node is not a noun.
# The singler action reduces the first three elements and leaves the lookahead element out of the reduction.
# When the rule matcher is done with the current rule match, it will move to the node that matched the lookahead element of the current rule.
# (Without the lookahead element modifier, the rule matcher normally would continue at the node after the one matching the 4th element of the rule.)
_xWILD [one lookahead fail=(_noun)] # Lookahead element.
trigger, Phrase Element Modifiers