support detecting which relaxed matches are lone emails

This doesn't require new API nor does it require us to do a second
regular expression match on each string, so I think it's the right way.
We can also add more subexpression names later which might be useful,
such as "web URL without scheme" or "URL scheme".

Updates #53.

example_test.go

@@ -27,3 +27,18 @@ func ExampleStrictMatchingScheme() {
 	// Output:
 	// [https://foo.com/dl]
 }
+
+func Example_filterEmails() {
+	s := "Email dev@foo.com about any issues with foo.com or https://foo.com/dl"
+	rx := xurls.Relaxed()
+	idxEmail := rx.SubexpIndex("relaxedEmail")
+	for _, match := range rx.FindAllStringSubmatch(s, -1) {
+		if match[idxEmail] != "" {
+			continue // skip lone email addresses
+		}
+		fmt.Println(match[0])
+	}
+	// Output:
+	// foo.com
+	// https://foo.com/dl
+}

xurls.go

@@ -164,7 +164,7 @@ func relaxedExp() string {
 
 	hostName := `(?:` + domain + `|\[` + ipv6Addr + `\]|\b` + ipv4Addr + `\b)`
 	webURL := hostName + port + `(?:/` + pathCont + `|/)?`
-	email := `[a-zA-Z0-9._%\-+]+@` + domain
+	email := `(?P<relaxedEmail>[a-zA-Z0-9._%\-+]+@` + domain + `)`
 	return strictExp() + `|` + webURL + `|` + email + `|` + ipv6AddrMinusEmpty
 }
 
@@ -179,7 +179,10 @@ func Strict() *regexp.Regexp {
 }
 
 // Relaxed produces a regexp that matches any URL matched by Strict, plus any
-// URL with no scheme or email address.
+// URL or email address with no scheme.
+//
+// Email addresses without a scheme match the `relaxedEmail` subexpression,
+// which can be used to filter them as needed.
 func Relaxed() *regexp.Regexp {
 	relaxedInit.Do(func() {
 		relaxedRe = regexp.MustCompile(relaxedExp())