I am trying to run a regex validation on the filename of a document uploaded to
a library on my sharepoint server. I've tried testing the validation on both
the NAME fields with no luck. My regex is as follows:

[a-zA-Z]_+[a-zA-Z]_[.]([a-zA-Z0-9]{4}|[a-zA-Z0-9]{3})

Valid filenames would have two underscores and a 3-4 character extension:

test_filename_.docx (valid)
test_filename_.xls (valid)
testfilename.xlsx (invalid)