Simplify validation pattern of EndpointId

See gh-19584

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/EndpointId.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -40,9 +40,9 @@ public final class EndpointId {
 
 	private static Set<String> loggedWarnings = new HashSet<>();
 
-	private static final Pattern VALID_PATTERN = Pattern.compile("[a-zA-Z0-9\\.\\-]+");
+	private static final Pattern VALID_PATTERN = Pattern.compile("[a-zA-Z0-9.-]+");
 
-	private static final Pattern WARNING_PATTERN = Pattern.compile("[\\.\\-]+");
+	private static final Pattern WARNING_PATTERN = Pattern.compile("[.-]+");
 
 	private final String value;
 

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/EndpointIdTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -51,6 +51,12 @@ public class EndpointIdTests {
 				.withMessage("Value must only contain valid chars");
 	}
 
+	@Test
+	public void ofWhenContainsBackslashThrowsException() {
+		assertThatIllegalArgumentException().isThrownBy(() -> EndpointId.of("foo\\bar"))
+				.withMessage("Value must only contain valid chars");
+	}
+
 	@Test
 	public void ofWhenHasBadCharThrowsException() {
 		assertThatIllegalArgumentException().isThrownBy(() -> EndpointId.of("foo!bar"))