This is a little suspicious: /(:?aaa)?/ ...because /(?:aaa)?/ would be non-capturing parentheses. What you have instead is a capturing parenthesis expression that begins with an optional colon. Is that what you meant?

As to your question, your entire expression is optional (i.e., /(aaa)?/), so it doesn't really require Perl to match anything. In my little fooling around, I didn't find a way to get it to find the optional string while still keeping it optional, but there probably is a way.