support mailto links (#911)

packages/expo-router/src/global-state/routing.ts

@@ -7,7 +7,7 @@ import * as Linking from "expo-linking";
 import { ResultState } from "../fork/getStateFromPath";
 import { Href, resolveHref } from "../link/href";
 import { resolve } from "../link/path";
-import { hasUrlProtocolPrefix } from "../utils/url";
+import { shouldLinkExternally } from "../utils/url";
 import type { RouterStore } from "./router-store";
 
 function assertIsReady(store: RouterStore) {
@@ -52,7 +52,7 @@ export function setParams(
 }
 
 export function linkTo(this: RouterStore, href: string, event?: string) {
-  if (hasUrlProtocolPrefix(href)) {
+  if (shouldLinkExternally(href)) {
     Linking.openURL(href);
     return;
   }

packages/expo-router/src/utils/url.ts

@@ -5,3 +5,19 @@
 export function hasUrlProtocolPrefix(href: string): boolean {
   return /^[\w\d_+.-]+:\/\//.test(href);
 }
+
+export function isWellKnownUri(href: string): boolean {
+  // This is a hack and we should change this to work like the web in the future where we have full confidence in the
+  // ability to match URLs and send anything unmatched to the OS. The main difference between this and `hasUrlProtocolPrefix` is
+  // that we don't require `//`, e.g. `mailto:` is valid and common, and `mailto://bacon` is invalid.
+  return /^(https?|mailto|tel|sms|geo|maps|market|itmss?|itms-apps|content|file):/.test(
+    href
+  );
+}
+
+export function shouldLinkExternally(href: string): boolean {
+  // Cheap check first to avoid regex if the href is not a path fragment.
+  return (
+    !/^[./]/.test(href) && (hasUrlProtocolPrefix(href) || isWellKnownUri(href))
+  );
+}