There is in fact a problem here, but not the one the OP was suggesting.
First (the non-problem): the regex "(?ism)order your.*?736.*?$" removes the footer on this document, assuming that one remembers to check the remove footer box.
Second (the problem): the regexp tester does not show multi-line matches. The problem is that it uses the QSyntaxHighlighter, which my experimentation shows to be a line-oriented interface, making highlighting multiline matches impossible. I think that the regex texter should match directly against the text in the QTextEdit box, using something like setTextBackgroundColor to indicate matches. I admit that I haven't hacked the code to try this idea, but it seems plausible. I will file a ticket on this so that someone more acquainted than I am with the widgets can think about a solution.