是的，这是一个错误(或者至少是一个错误功能).它抱怨如果 a* 什么都不匹配，它就不知道如何捕获 0 个或多个无".

Yes, it's a bug (or at least a misfeature). It's complaining that if a* matches nothing, it doesn't know how to capture 0 or more "nothings".