lookahead, lookahead not


lookahead pattern
lookahead not pattern


You can use lookahead to match a pattern without consuming it:

     submit "Mary had a little lamb%n"
  find " " lookahead "little"
     output " tiny "
This program outputs Mary had a tiny little lamb. Without lookahead, it would output Mary had a tiny lamb.

lookahead must be applied only to part of the pattern of a find rule. If the entire rule were lookahead, no characters would be consumed and so the rule would be matched an infinite number of times. In the program above, matching the space before little ensures that this condition is met.

You can also match any pattern which is not followed by a certain pattern using lookahead not. This program changes the size of all the animals except the lamb.

     submit "Mary had a little lamb%n"
     submit "Mary had a little horse%n"
     submit "Mary had a little pig%n"
  find "little " lookahead not "lamb"
     output "big "

Related Syntax
Related Concepts