Regex Match Ampersand but not escaped xml characters

Question

I would like to match ampersand (&) but not when it exists in following manner

'
"
>
<
&
&#

Answer 1

That looks like a job for negative lookahead assertions:

&(?!(?:apos|quot|[gl]t|amp);|#)

should work.

Explanation:

&        # Match &
(?!      # only if it's not followed by
 (?:     # either
  apos   # apos
 |quot   # or quot
 |[gl]t  # or gt/lt
 |amp    # or amp
 );      # and a semicolon
|        # or
 \#      # a hash
)        # End of lookahead assertion