FIX - Catastrophic Backtracking for images with illegal char in tag

- Simplify regular expression checking for validity of Image Reference to fix Catastrophic Backtracking for long custom images with illegal characters in tag

#39246

spring-boot-project/spring-boot-docker-compose/src/main/java/org/springframework/boot/docker/compose/core/Regex.java

@@ -50,7 +50,7 @@ final class Regex implements CharSequence {
 	private static final Regex PATH_COMPONENT;
 	static {
 		Regex segment = Regex.of("[a-z0-9]+");
-		Regex separator = Regex.group("[._]|__|[-]*");
+		Regex separator = Regex.group("[._-]*[a-z0-9]+");
 		Regex separatedSegment = Regex.group(separator, segment).oneOrMoreTimes();
 		PATH_COMPONENT = Regex.of(segment, Regex.group(separatedSegment).zeroOrOnce());
 	}

spring-boot-project/spring-boot-docker-compose/src/test/java/org/springframework/boot/docker/compose/core/ImageReferenceTests.java

@@ -16,10 +16,14 @@
 
 package org.springframework.boot.docker.compose.core;
 
+import java.time.Duration;
+
 import org.junit.jupiter.api.Test;
+import org.testcontainers.shaded.org.awaitility.Awaitility;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+import static org.assertj.core.api.Assertions.fail;
 
 /**
  * Tests for {@link ImageReference}.
@@ -165,4 +169,20 @@ void equalsAndHashCode() {
 		assertThat(r1).isEqualTo(r1).isEqualTo(r2).isNotEqualTo(r3);
 	}
 
+
+	@Test
+	void ofWhenImageNameIsVeryLongAndHasIllegalCharacter() {
+		Awaitility.await()
+				.timeout(Duration.ofSeconds(2))
+				.until(() -> {
+					try {
+						ImageReference.of("docker.io/library/this-image-has-a-long-name-with-an-invalid-tag-which-is-at-danger-of-catastrophic-backtracking:1.0.0+1234");
+						fail("Image Reference contains an illegal character and should have thrown an IllegalArgumentException");
+					}
+					catch (IllegalArgumentException ignored) {
+					}
+					return true;
+				});
+	}
+
 }