Early on in post 1 it starts with "What on earth is a regular expression?
" and by the third sentence we're into case, with lots of discussion about case flags in the discussion that follows.
This is a logical place to start that discussion in a general discussion of regexps. A general regexp is case sensitive, and it's an important concept, but as Charles posted about Calibre:
ignore case is turned on by default, and therefore cannot be turned off.
This can be pretty confusing if someone is trying to understand regex matching by doing actual searches on the search bar. You say "A" does not match "a" but in Calibre's search bar it does, even if (?i) is not set. If they try to do regexes in the new bulk metadata edit page, they don't use the flags, they use an option switchbox.
Since this discussion is specific to Calibre, I wonder if it would be worth stating up front where ignore case is turned on by default and where it can be controlled by selecting an option box.