Got rid of unnecessary special case to match at end-of-string

Instead, I tweaked the rest of the matching function, so that a special
check isn't necessary. If we are trying to match at the end of a string,
we skip any of the actual matching and proceed straight to finding
0-length matches.

This change was made because, with the special case, capturing groups
weren't getting updated if we had an end-of-string match.

matching.go

@@ -60,8 +60,7 @@ func (g Group) isValid() bool {
 // takeZeroState takes the 0-state (if such a transition exists) for all states in the
 // given slice. It returns the resulting states. If any of the resulting states is a 0-state,
 // the second ret val is true.
-// The third ret val is a list of all the group numbers of all the opening parentheses we crossed,
+// If a state begins or ends a capturing group, its 'thread' is updated to contain the correct index.
-// and the fourth is a list of all the closing parentheses we passed
 func takeZeroState(states []*State, numGroups int, idx int) (rtv []*State, isZero bool) {
 	for _, state := range states {
 		if len(state.transitions[EPSILON]) > 0 {
@@ -169,23 +168,6 @@ func findAllMatchesHelper(start *State, str []rune, offset int, numGroups int) (
 		// The second value here shouldn't be used, because we should exit when the third return value is > than len(str)
 		return false, []Group{}, offset
 	}
-	// 'Base case' - if we are at the end of the string, check if we can add a zero-length match
-	if offset == len(str) {
-		// Get all zero-state matches. If we can get to a zero-state without matching anything, we
-		// can add a zero-length match. This is all true only if the start state itself matches nothing.
-		// TODO - fill in capturing groups for these matches
-		if start.isEmpty {
-			to_return := newMatch(numGroups + 1)
-			if start.groupBegin {
-				to_return[start.groupNum].startIdx = offset
-			}
-			if ok := zeroMatchPossible(str, offset, numGroups, start); ok {
-				to_return[0] = Group{offset, offset}
-				return true, to_return, offset + 1
-			}
-		}
-		return false, []Group{}, offset + 1
-	}
 
 	// Hold a list of match indices for the current run. When we
 	// can no longer find a match, the match with the largest range is
@@ -366,7 +348,7 @@ func findAllMatchesHelper(start *State, str []rune, offset int, numGroups int) (
 	for _, state := range currentStates {
 		// Only add the match if the start index is in bounds. If the state has an assertion,
 		// make sure the assertion checks out.
-		if state.isLast && startIdx < len(str) {
+		if state.isLast && i <= len(str) {
 			if state.assert == NONE || state.checkAssertion(str, i) {
 				for j := 1; j < numGroups+1; j++ {
 					tempIndices[j] = state.threadGroups[j]