You can do it without backtracking with a very simple deterministic finite automaton - keep track of <block_start, block_end, condition_met> as you pour through

You can do it without backtracking with a very simple deterministic finite automaton - keep track of <block_start, block_end, condition_met> as you pour through the input. But yes, it's a bit more complicated, it's preferable to just have one line per "log item"