Why is this preg_match not working?

That's what's happening!

When - and only when - the form field being validated contains, anyhwere within it, an ampersand followed immediately by two or more characters (e.g. foo&ba or foo&bar), a semi-colon is being added to the end of the field value passed to the callback function.

So, for example
- User enters "foo&b" and the callback function is passed "foo&b" to validate.
- User enters "foo&bar" and the callback function is passed "foo&bar;" to validate.

Obviously, I can workaround this now. I have no idea why it's happening though! I'm using Grocery CRUD with Codeigniter so the explanation is probably buried deep inside one of those frameworks.

Thanks for your help.