support replacing http://skins.minecraft.net/MinecraftSkins/%s.png

see #7 for details

src/main/java/org/to2mbn/authlibinjector/transform/YggdrasilApiTransformUnit.java

@@ -1,12 +1,23 @@
 package org.to2mbn.authlibinjector.transform;
 
+import java.util.function.Function;
 import java.util.regex.Pattern;
 
 public class YggdrasilApiTransformUnit extends RegexTransformUnit {
 
+	// TODO: If java.util.regex supports branch reset, we can use the following regex:
+	// ^https?:\/\/(?|((?<subdomain>(api|authserver|sessionserver))\.(?<domain>mojang\.com))|((?<subdomain>(skins))\.(?<domain>minecraft\.net)))(?<path>\/.*)$
+	// => ${apiRoot}${subdomain}${path}
+	//
+	// But now it's not supported, so we use a workaround here:
+	// ^https?:\/\/(?:(?:(api|authserver|sessionserver)\.(mojang\.com))|(?:(skins)\.(minecraft\.net)))(?<path>\/.*)$
+	// => ${apiRoot}$1$3${path}
+
+	public static final Pattern REGEX = Pattern.compile("^https?:\\/\\/(?:(?:(api|authserver|sessionserver)\\.(mojang\\.com))|(?:(skins)\\.(minecraft\\.net)))(?<path>\\/.*)$");
+	public static final Function<String, String> REPLACEMENT = apiRoot -> apiRoot + "$1$3${path}";
+
 	public YggdrasilApiTransformUnit(String apiRoot) {
-		// ^https:\/\/(api|authserver|sessionserver)\.mojang\.com\/(.*)$
+		super(REGEX, REPLACEMENT.apply(apiRoot));
-		super(Pattern.compile("^https:\\/\\/(api|authserver|sessionserver)\\.mojang\\.com\\/(.*)$"), apiRoot + "$1/$2");
 	}
 
 	@Override

src/test/java/org/to2mbn/authlibinjector/test/UrlReplaceTest.java

@@ -0,0 +1,98 @@
+package org.to2mbn.authlibinjector.test;
+
+import static org.junit.Assert.assertEquals;
+import java.util.Arrays;
+import java.util.Collection;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+import org.junit.runners.Parameterized.Parameter;
+import org.junit.runners.Parameterized.Parameters;
+import org.to2mbn.authlibinjector.transform.YggdrasilApiTransformUnit;
+
+@RunWith(Parameterized.class)
+public class UrlReplaceTest {
+
+	private static final String apiRoot = "https://yggdrasil.example.com/";
+
+	@Parameters
+	public static Collection<Object[]> data() {
+		// @formatter:off
+		return Arrays.asList(new Object[][] {
+			{
+				// from: [com.mojang:authlib:1.5.24]/com.mojang.authlib.yggdrasil.YggdrasilGameProfileRepository
+				"https://api.mojang.com/profiles/",
+				"https://yggdrasil.example.com/api/profiles/"
+			},
+			{
+				// from: [com.mojang:authlib:1.5.24]/com.mojang.authlib.yggdrasil.YggdrasilMinecraftSessionService
+				"https://sessionserver.mojang.com/session/minecraft/join",
+				"https://yggdrasil.example.com/sessionserver/session/minecraft/join"
+			},
+			{
+				// from: [com.mojang:authlib:1.5.24]/com.mojang.authlib.yggdrasil.YggdrasilMinecraftSessionService
+				"https://sessionserver.mojang.com/session/minecraft/hasJoined",
+				"https://yggdrasil.example.com/sessionserver/session/minecraft/hasJoined"
+			},
+			{
+				// from: [com.mojang:authlib:1.5.24]/com.mojang.authlib.yggdrasil.YggdrasilUserAuthentication
+				"https://authserver.mojang.com/authenticate",
+				"https://yggdrasil.example.com/authserver/authenticate"
+			},
+			{
+				// from: [com.mojang:authlib:1.5.24]/com.mojang.authlib.yggdrasil.YggdrasilUserAuthentication
+				"https://authserver.mojang.com/refresh",
+				"https://yggdrasil.example.com/authserver/refresh"
+			},
+			{
+				// from: [com.mojang:authlib:1.5.24]/com.mojang.authlib.yggdrasil.YggdrasilUserAuthentication
+				"https://authserver.mojang.com/validate",
+				"https://yggdrasil.example.com/authserver/validate"
+			},
+			{
+				// from: [com.mojang:authlib:1.5.24]/com.mojang.authlib.yggdrasil.YggdrasilUserAuthentication
+				"https://authserver.mojang.com/invalidate",
+				"https://yggdrasil.example.com/authserver/invalidate"
+			},
+			{
+				// from: [com.mojang:authlib:1.5.24]/com.mojang.authlib.yggdrasil.YggdrasilUserAuthentication
+				"https://authserver.mojang.com/signout",
+				"https://yggdrasil.example.com/authserver/signout"
+			},
+			{
+				// from: [mcp940]/net.minecraft.client.entity.AbstractClientPlayer
+				// issue: to2mbn/authlib-injector#7 <https://github.com/to2mbn/authlib-injector/issues/7>
+				"http://skins.minecraft.net/MinecraftSkins/%s.png",
+				"https://yggdrasil.example.com/skins/MinecraftSkins/%s.png"
+			},
+			{
+				// from: [bungeecord@806a6dfacaadb7538860889f8a50612bb496a2d3]/net.md_5.bungee.connection.InitialHandler
+				// url: https://github.com/SpigotMC/BungeeCord/blob/806a6dfacaadb7538860889f8a50612bb496a2d3/proxy/src/main/java/net/md_5/bungee/connection/InitialHandler.java#L409
+				"https://sessionserver.mojang.com/session/minecraft/hasJoined?username=",
+				"https://yggdrasil.example.com/sessionserver/session/minecraft/hasJoined?username="
+			},
+			{
+				// from: [wiki.vg]/Mojang_API/Username -> UUID at time
+				// url: http://wiki.vg/Mojang_API#Username_-.3E_UUID_at_time
+				// issue: to2mbn/authlib-injector#6 <https://github.com/to2mbn/authlib-injector/issues/6>
+				"https://api.mojang.com/users/profiles/minecraft/",
+				"https://yggdrasil.example.com/api/users/profiles/minecraft/"
+			}
+		});
+		// @formatter:on
+	}
+
+	@Parameter(0)
+	public String input;
+
+	@Parameter(1)
+	public String output;
+
+	@Test
+	public void test() {
+		assertEquals(output,
+				YggdrasilApiTransformUnit.REGEX.matcher(input).replaceAll(
+						YggdrasilApiTransformUnit.REPLACEMENT.apply(apiRoot)));
+	}
+
+}