Find in Bookview seems to start the search one character before the cursor. I think this may be part of the cache issue you discussed and not something to be too concerned with since its Bookview and works in Codeview ok, but I'll mention it anyway.
Enter "for testing", put the cursor between t and e. Do a find for testing - it will find testing even though its after the t.
Switch to Regex, put the cursor between the space and t. Do a find for ".testing" - it will highlight the space and the word testing instead of not finding anything.
Also, issue 1159 also exists on Linux - highlight text in BV, switch to CV and its been deleted (still available in BV undo)
See the Sigil User Guide and its tutorials for details about Sigil.