Lookaheads, denoted by the & and ! characters, can be used to check the next piece of input without consuming it. The expression &expr matches if the next piece of input matches expr, and !expr matches if the input does not match expr.

For example, this rule matches the word television followed by a colon, but does not consume the colon:

tv   <-  "television" &":"

This rule matches any number of non-space characters, by first checking the next character using a negative lookahead and then consuming it with the wildcard operator:

nonspace   <-  (!" " .)*

The lookahead operators do not have to be followed by strings, they can be used with any other type of parsing expression.