demonstrating that lexing JavaScript requires a pushdown automaton

explanation.txt

Looking at the preceding token when the lexer points to a '/'
character is insufficient to determine which context it's in. In
fact, you need to look at an arbitrary number of preceding tokens
to figure it out. This example demonstrates a case where we can
pump up the number of preceding tokens to an arbitrary size
before you can disambiguate your syntactic context.

lexability1.js

Function.prototype.valueOf = function() { return 100; };
Number.prototype.test = function() { return true; };

var foo = 2, i = 5;
//                ^
// Have to look back to here to figure out
// that we're in an operand position.

function f() { /* arbitrary body */ } /foo/i.test('') && console.log("hello world!")
//                                    ^
//                          lexer is pointing here

;

lexability2.js

Function.prototype.valueOf = function() { return 100; };
Number.prototype.test = function() { return true; };

var foo = 2, i = 5; (
//                  ^
// Have to look back to here to figure out
// that we're in an operator position.

function f() { /* arbitrary body */ } /foo/i.test('') && console.log("hello world!")
//                                    ^
//                          lexer is pointing here

);