Positive Lookahead:

The positive lookahead asserts that the fist part of the expression must be followed by the lookahead expression. (T|t)he(?=\sfat) Match The or the only if it's followed by the word fat.

Negative Lookahead:

Get all the matches that are not followed by a certain pattern. (T|t)he(?!\sfat) which means get all The or the words from the input string that are not followed by a space character and the word fat.

Positive Lookbehind

Used to get all the matches that are preceded by a specific pattern. (?<=(T|t)he\s)(fat|mat) means: get all fat or mat words from the input string that come after the word The or the.

Negative Lookbehind

Used to get all the matches that are not preceded by a specific pattern. (?<!(T|t)he\s)(cat) means: get all cat words from the input string that are not after the word The or the.