When performing a search with regular expressions, Opus crashes. Is there something basic I am missing that might cause this? It happens no matter what I enter as criteria. It does not happen however when I use my pre-defined filters that utilize regular expressions only when I enter one on the fly. Attached is an example screenshot. By the way, when it crashes, I only get the "Do you want to debug" dialog and is the only indication of the crash that is visible.
Assuming you are looking for a file containing the word "StringBuilder", there is nothing wrong with your search parameters. In the screen grab you posted, you do not actually need to check Regular Expression, as you are not really using any.
I just performed a similar search here without issue (with the same options enabled). So something must be wrong specific to your: PC, OS, or Opus installation.
Try creating a an empty test folder. Then create one text file inside it with a search word. Then try to find that file using a similar search, and see if Opus crashes. If it doesn't, there might be a corrupt or locked file in the folder you were searching in.