regexp: example for MatchString function

Change-Id: I5ca5a6689f0679154c24820466f5cf0011d0aaa6
Reviewed-on: https://go-review.googlesource.com/48959
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/regexp/example_test.go

@@ -109,6 +109,17 @@ func ExampleRegexp_FindAllStringSubmatchIndex() {
 	// []
 }
 
+func ExampleRegexp_MatchString() {
+	re := regexp.MustCompile("(gopher){2}")
+	fmt.Println(re.MatchString("gopher"))
+	fmt.Println(re.MatchString("gophergopher"))
+	fmt.Println(re.MatchString("gophergophergopher"))
+	// Output:
+	// false
+	// true
+	// true
+}
+
 func ExampleRegexp_ReplaceAllLiteralString() {
 	re := regexp.MustCompile("a(x*)b")
 	fmt.Println(re.ReplaceAllLiteralString("-ab-axxb-", "T"))