1) abababab:
The pattern (ab)+ captures one or more occurrences of the sequence "ab". The group(0) retrieves the entire matched substring.

2) ['a', 'aaa', 'aaa', 'aa']
The a{1,3} matches all occurences of "a" that are repeated 1 to 3 times

3) ['apple']
\b is a word boundary, but the important thing ist the (?=) which is a so called lookahead!
4) Its an email validator!