Someone didn't escape their & in their HTML correctly. Let's fix it.

This regexp correctly does not escape &dude, because we only want to escape
raw & markers:

  p "yo &dude".gsub(/&([^a-z])/i, '&\1')

That passed "yo &dude" thru unchanged. (I am aware "dude" has no ; on the
end; we are leaving that optional, for whatever reason...)

Now escape & followed by a non-alphabetic character:

  p "yo & dude".gsub(/&([^a-z])/i, '&\1')

That correctly provides: "yo & dude"

Now how to escape "yo && dude"? Note that the ([^a-z]) consumes the second
&, leading to this incorrect output:

  "yo && dude"

The only workaround I can think of is to run the Regexp twice:

  x = "yo && dude"
  2.times{ x.gsub!(/&([^a-z])/i, '&\1') }
  p x

Can someone help my feeb Regexp skills and get a "yo && dude" in one
line?

The regular expression trick here is the (?=re)  That's called the
"zero-width positive lookahead". It matches, but it does not consume
the string; so the gsub! will only replace the characters that are NOT
inside (?=re).