Looking ahead

notFollowedBy

Say you want to match the print keyword in a programming language. You can express that with the string("print") parser, but it will match more than you'd like:

<?php

$print = string("print");

$result = $print->tryString("print('Hello World');");

assertEquals("print", $result->output());

$result = $print->tryString("printXYZ('Hello World');");

assertEquals("print", $result->output()); // oops!

As you can see, "printXYZ" also results in "print", but it wasn't our intention, because "printXYZ" is not a valid keyword.

We can solve it by using the notFollowedBy combinator.

<?php

$print = keepFirst(string("print"), notFollowedBy(alphaNumChar()));

$result = $print->run(new StringStream("printXYZ('Hello World');"));

assertTrue($result->isFail());

There's a fluent interface as well:

<?php

$print = string("print")->notFollowedBy(alphaNumChar());

$result = $print->run(new StringStream("printXYZ('Hello World');"));

assertTrue($result->isFail());

In practice, we'll have a lot more keywords than just the one. A good habit is to first generalize this to all the keywords in our language. Then, using our new $keyword parser constructor, we can match the exact variations we like:

<?php

$keyword = fn(string $name) => keepFirst(string($name), notFollowedBy(alphaNumChar()));

$parser = choice(

$keyword('printf'),

$keyword('print'),

$keyword('sprintf')

);

$result = $parser->tryString("print('Hello World');");

assertEquals("print", $result->output());

$result = $parser->tryString("printf('Hello %s', 'world');");

assertEquals("printf", $result->output());