I have one more Find & Replace enhancement suggestion: how about adding a simple regex validator?
Apparently, you can actually
use a regular expression for this.
But I'm sure that there must be some Open Source C++ libraries for validating regular expressions.
(Since Auto-Tokenize automatically escapes some characters, users shouldn't be able to select both Auto-Tokenize and Validate Regex.)
When Validate Regex is checked and the Find/Replace boxes contain invalid regular expressions, Sigil should display a warning, when the user clicks the Find button. Maybe you could also format invalid regular expressions in red to indicate that they're invalid.