If you don't want to use File::Basename or File::Spec, you still shouldn't use regexen. Why use a complicated machine which many parameters, when what you want is a salami-slicer?

If you don't use the clean and simple tools, use split. Of course you have to decipher whether to split on '/' for Macs or unix, '\' for Windows, was it ':' on VMS?, or something else ... all that is handled for you if you use the right tool. After you split, just take the last component and that's the filename.