Its not just a regex option, when the `multiline` option is clear Geany actually searches line by line.
This is also a premature optimization since it can save having to coalesce the Scintilla buffer which can result in moving the entire document in memory if the cursor is near the start of a document. If thats really an issue requires benchmarking, but its likely to only apply to _very_ large documents (and whats large depends on your machine).
As to which should be the default, myeh.