RegEx to Reject Unescaped HTML Character

Question

I want to restrict usage of unescaped ampersands in a particular input field. I\'m having trouble getting a RegEx to kill usage of \"&\" unless followed by \"amp;\"...or per

Answer 1

You can match on /&(?!amp;)/ to locate any &'s not followed by &. The (?!) construction is called a negative lookahead.

Assuming you're using a regexp engine that supports them, at any rate. I know Perl/PCRE regexps do.