BUG: fix event regex

I noticed that random strings (e.g., from git hashes) can match the existing regex pattern for finding events. I refined the query to match exactly either long-form event names GWYYMMDD_SSSSSS or superevent ID SYYMMDDabc.

This will make it ignore file names using the short form of event names, e.g., GWYYMMDD, I'm not sure if I should relax this. Edit: I relaxed this.