Steps to reproduce:

1. Create Item
2. Add Regex Preprocessing:
   1. Pattern: ">(ESX[^<]).\n.>(\d\/\d+\/\d+).\n.>(\d+)"
   2. Replacement: {"name":"\1","date":"\2","build":"\3"}
3. Click on Test

Result:
See screenshot:

It's not possible to modify or test the item, while the item still gathers and saves the valid preprocessed values.

Expected:
Test and save should work normally, since the regular expression is valid.

Additional Info:

The problem was not present in 7.2.x. I cannot say for sure, if it was also present in 7.4.0.