regexp: add example for NumSubexp

Updates #21450

Change-Id: Idf276e97f816933cc0f752cdcd5e713b5c975833
GitHub-Last-Rev: 198e585f92db6e7ac126b49cd751b333e9a44b93
GitHub-Pull-Request: golang/go#33490
Reviewed-on: https://go-review.googlesource.com/c/go/+/189138
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/regexp/example_test.go

@@ -181,6 +181,13 @@ func ExampleRegexp_MatchString() {
 	// true
 }
 
+func ExampleRegexp_NumSubexp() {
+	re := regexp.MustCompile(`(.*)((a)b)(.*)a`)
+	fmt.Println(re.NumSubexp())
+	// Output:
+	// 4
+}
+
 func ExampleRegexp_ReplaceAll() {
 	re := regexp.MustCompile(`a(x*)b`)
 	fmt.Printf("%s\n", re.ReplaceAll([]byte("-ab-axxb-"), []byte("T")))