I have a document library in which I would like to filter based on a substring
of the file name. Basically, I'm wanting to only show the documents in which
there is a underscore for the fifth letter of the "Name" field.

I created a document library view, opened in SPD and converted it to a data
view.
I clicked on Filter and then checked ADD XSLT filtering and clicked the Edit
button.

I added the following code to the filter:

[substring(/dsQueryResponse/Rows/Row/@FileLeafRef,5,1)='_']

I click Ok and get the following message:
Failed setting processor stylesheet: 0x80004005: Expression must evaluate to a
node set.

Could someone help me with this? I'm not that familiar with doing advanced
coding in a dataview so I'm not sure what I'm doing wrong.