Some common usages of (?<=...), (?=...) and (?!...):

s/(?<=a)b/c/;       # Changes the 'a', but keeps the 'b'.

s/b(?=a)/c/;        # Changes the 'a', but keeps the 'b'.
/^(?=.*a)(?=.*b)/;  # Is equivalent to /a/ && /b/.

/a(?:(?!bcd).)*e/;  # (?:(?!...).)* is to regexp what
                    # [^...] is to characters.
                    # This example reads as:
                    # "Match 'a' followed by somethng which
                    # doesn't contain 'bcd', followed by 'e'."
[download]

$& refers to "everything that was matched". It's what gets substituted in a substitution. Avoid actually using $&. It has side effects which can slow down other regexps in your program. It can easily be emulated using captures.