negative lookbehind javascript

(?=foo)	Lookahead	Asserts that what immediately follows the current position in the string is foo
(?<=foo)	Lookbehind	Asserts that what immediately precedes the current position in the string is foo
(?!foo)	Negative Lookahead	Asserts that what immediately follows the current position in the string is not foo
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Here is what the above code is Doing:
1. The first line of the code defines a regular expression that matches a string with exactly six characters.
2. The second line of the code defines a regular expression that matches a string with exactly six characters and the first three of those characters are world.
3. The third line of the code defines a regular expression that matches a string with exactly six characters and the last three of those characters are world.
4. The fourth line of the code defines a regular expression that matches a string with exactly six characters and the first three of those characters are not world.
5. The fifth line of the code defines a regular expression that matches a string with exactly six characters and the last three of those characters are not world.