[^\n\r]*/([^/\n\r]*)/[^/\n\r]*

Match 0 or more characters except carriage returns or line feeds,
Followed by a single / character,
Next match 0 or more characters except / characters, carriage returns or line feeds again, group this for later retrieval,
Match another single /
Finally, another match of 0 or more chars except / returns and feeds as before.

In english: extract the second to last string seperated by / from a line.

(\d\d\d\d/\d\d/\d\d/[-_\.\w\d]*?)

Match an 8 digit number, followed by anything consiting of - _ . letters or numbers, finally followed by a ?