regex (in PHP) to match & that aren't HTML entities

Question

Here's the goal: to replace all standalone ampersands with & but NOT replace those that are already part of an HTML entity such as  .

I think I need a regular expression for PHP (preferably for preg_ functions) that will match only standalone ampersands. I just don't know how to do that with preg_replace.

Answer 1

PHP's htmlentities() has double_encode argument for this.

If you want to do things like that in regular expressions, then negative assertions come useful:

preg_replace('/&(?!(?:[[:alpha:]][[:alnum:]]*|#(?:[[:digit:]]+|[Xx][[:xdigit:]]+));)/', '&', $txt);