Hi there,

I believe there's a possible bug with regex replaces that use backreference when the regex contains lookahead/lookbehind.

What I'm attempting to do is find all instances of & in a URL and replace it with & for w3c compliance.

The regex I came up with is:

(?<=[\w])&((?!amp;|nbsp;)[\w$]+?)(?==)

the replace sequence I use is:

&amp;\1

The result is that the replace actually literally inserts:

&amp;\1.

For instance in the string:

http://mywebsite/index.php?id=1&f=3

the result is:

http://mywebsite/index.php?id=1&amp\1=3


PS. Perhaps my regex sucks, but I know for sure regex coach (http://weitz.de/regex-coach/) confirms my regex should be working fine. I know the regex is capable of matching the & because I have tested replaces without the backreference and they work fine.

yep, looks like something with regexp.
I just tried some simple ones and they worked fine.
What regexps we support are based on PCRE 4.5 library.

After a little investigation it appeared that regexp library we use works fine. The problem was in our wrapping stuff and it's fixed by now and your sample gets:
http://mywebsite/index.php?id=1f=f=3
as it should (\0 contains &f and \1 contains f).

Fix will be available in a next few days.

This is great news!!