Wrote 'reg.FindSubmatch()' which returns the leftmost match with submatches, renamed 'isValid' to 'IsValid' to export it, renamed 'ToString' to 'String'

cmd/main.go

@@ -137,7 +137,7 @@ func main() {
 					fmt.Fprintf(out, "Line %d:\n", lineNum)
 				}
 				for _, m := range matchIndices {
-					fmt.Fprintf(out, "%s\n", m.ToString())
+					fmt.Fprintf(out, "%s\n", m.String())
 				}
 				err := out.Flush()
 				if err != nil {

regex/matching.go

@@ -35,10 +35,10 @@ func (m Match) numValidGroups() int {
 }
 
 // Returns a string containing the indices of all (valid) groups in the match
-func (m Match) ToString() string {
+func (m Match) String() string {
 	var toRet string
 	for i, g := range m {
-		if g.isValid() {
+		if g.IsValid() {
 			toRet += fmt.Sprintf("Group %d\n", i)
 			toRet += g.toString()
 			toRet += "\n"
@@ -52,8 +52,9 @@ func (idx Group) toString() string {
 	return fmt.Sprintf("%d\t%d", idx.StartIdx, idx.EndIdx)
 }
 
-// Returns whether a group contains valid indices
+// Returns whether a group is valid (ie. whether it matched any text). It
-func (g Group) isValid() bool {
+// simply ensures that both indices of the group are >= 0.
+func (g Group) IsValid() bool {
 	return g.StartIdx >= 0 && g.EndIdx >= 0
 }
 
@@ -174,6 +175,20 @@ func (regex Reg) FindString(str string) string {
 	return str[zeroGroup.StartIdx:zeroGroup.EndIdx]
 }
 
+// FindSubmatch returns the leftmost match of the regex in the given string, including
+// the submatches matched by capturing groups. The returned [Match] will always contain the same
+// number of groups. The validity of a group (whether or not it matched anything) can be determined with
+// [Group.IsValid], or by checking that both indices of the group are >= 0.
+// The second-return value is nil if no match was found.
+func (regex Reg) FindSubmatch(str string) (Match, error) {
+	match, err := regex.FindNthMatch(str, 1)
+	if err != nil {
+		return Match{}, fmt.Errorf("no match found")
+	} else {
+		return match, nil
+	}
+}
+
 // FindAllString is the 'all' version of FindString.
 // It returns a slice of strings containing the text of all matches of
 // the regex in the given string.
@@ -372,7 +387,7 @@ func findAllSubmatchHelper(start *nfaState, str []rune, offset int, numGroups in
 		// Check if we can find a zero-length match
 		if foundPath == false {
 			if ok := zeroMatchPossible(str, i, numGroups, currentStates...); ok {
-				if tempIndices[0].isValid() == false {
+				if tempIndices[0].IsValid() == false {
 					tempIndices[0] = Group{startIdx, startIdx}
 				}
 			}
@@ -382,7 +397,7 @@ func findAllSubmatchHelper(start *nfaState, str []rune, offset int, numGroups in
 			startIdx++
 			//	i++
 			//			}
-			if tempIndices.numValidGroups() > 0 && tempIndices[0].isValid() {
+			if tempIndices.numValidGroups() > 0 && tempIndices[0].IsValid() {
 				if tempIndices[0].StartIdx == tempIndices[0].EndIdx { // If we have a zero-length match, we have to shift the index at which we start. Otherwise we keep looking at the same paert of the string over and over.
 					return true, tempIndices, tempIndices[0].EndIdx + 1
 				} else {