Good question. This was discussed many years ago in these forums somewhere, but I am unable to find it now. Probably in the previously corrupted version of the TechSupport forum.
What you enter into the Comments field in the View Sightings Setup window is put into an SQL query with a LIKE clause. For example, if you put "x" into the comments field, the SQL built and run looks like
Comments LIKE '*x*'
* = Any zero or more characters
So, any-or-no characters followed by "x" followed by any-or-no characters.
With that said, MSAccess (the Birder's Diary database engine) can use the rather limited syntax found here with the LIKE predicate. Would be nice if it supported full REGEX functionality, but it does not.
As an example, you can search for any numerical digit preceded by "x" by entering this into the search: "x[0-9]" or "x#".
Note: remember that BD puts an asterisk on the front and end of whatever you enter into the search field for Comments.
Let me know if I can assist further with any questions on this.