I am interpreting "entries" to mean records.
make a back up copy of your file and try out these two tools: Find/Replace and Replace Field contents. Both have pretty good documentation in the help file.
"Also, is there a way that I can arbitrarily pick groups of entries and delete them at once without having to figure out an arbitrary search function that would select them all?
Thanks a lot for your help!"
Try one of these methods:
1) Make sure all records are in your found set by selecting "Show all records" from the record menu.
Select each record you want to delete and omit it by selecting Omit Record from the Records menu or use the keyboard shortcut that works for your platform.
After you've omitted each such record, select Show Omitted Only from the Records menu.
Now select "Delete found records"
2) Define an extra field and put it on your layout. As you find a record or group of records you want to delete, enter a value unique to this group in the field of each of these records. Peform a find to find all such records you've marked for deletion and use "Delete Found Records" to delete them.