A yes-or-no answer isn't enough for real work - you need to know where the match is. Find returns the start and end offsets of the leftmost match, which every later query builds on.
Add Find, returning the start and end byte offsets of the leftmost match, or nil if there is none.
Search tools, editors, and tokenizers all need the location of a match, not just
its existence. Find scans start positions from left to right and returns the offsets
of the first one that matches - [2, 5] for b+ in aabbbc, meaning “bytes 2 up to
5”. Leftmost is the rule: for a in banana, it reports the a at index 1, the
earliest one, and stops.
This is where you reach back for the backtracking matcher rather than the NFA. The backtracker walks the text position by position, so it already knows where it is; you just need it to report where a successful match ended. The usual way is to have the matcher call back (or set a field) at the moment the pattern’s nodes are all satisfied, recording the current text offset as the end. That “tell me where you finished” hook is exactly what tomorrow’s capture groups extend - they record where each parenthesized sub-part finished, too.
// Reuse the BACKTRACKING matcher - it knows positions.// Try start positions left to right; at the first start that// matches, capture where the match ended (thread an "end" out of// the matcher when the pattern's nodes are exhausted).// Return []int{start, end}; nil if no start works.