(x) Matches 'x' and remembers the match, as the following example shows. The parentheses are called capturing parentheses. The '(foo)' and '(bar)' in the pattern /(foo) (bar) \1 \2/ match and remember the first two words in the string "foo bar foo bar". The \1 and \2 in the pattern match the string's last two words.
I want to know if this pattern
/(foo) (bar) \1 \2/ is in fact a RE according the definition of RE that we have in theoretical formal language or it is something more powerful. And if it is so I would like to know if this kind of feature is implemented also with FA or in another way (in particualr how it is implemented).