Regex redirection match string inside url

Hi,
Setup the following for a redirection
Source url: /au[/]?$
Regex: Y
Target URL: /event/cloudsec-australia-2016/
HttpCode: 307 – Temporary Redirect

This works when user typed the following and get redirected to the target.
https://www.mydomain.com/au
https://www.mydomain.com/au/

I have an issue that the regex also matches within the source url
e.g. the following will also be redirected to the target
https://www.mydomain.com/survey/au/

https://ww.wp.xz.cn/plugins/redirection/