Bash script to iterate files in directory and pattern match filenames

The easiest way is probably just to iterate each group separately. This side-steps the parsing issue entirely.

DIRECTORY=.

for i in $DIRECTORY/YYYYMMDD_*_bulk_import.csv; do
    # Process $i
done

for i in $DIRECTORY/YYYYMMDD_*_genstats_import.csv; do
    # Process $i
done

for i in $DIRECTORY/YYYYMMDD_*allstats.csv; do
    # Process $i
done

Set DIRECTORY to whatever directory you want to search. The default . will search the current working directory.