Fix invalid prefix when generating spring boot config (#1862)

Fix invalid prefix when generating spring boot config

In Spring Boot, the prefix passed to `@ConfigurationProperties` must
be in "canonical form".

The presence of this annotation allows spring boot to directly inject
properties into a class, but is not actually needed.

The correct behavior here is to omit this annotation if the property
name does not match Spring Boot's canonical form.

pkl-commons/src/main/kotlin/org/pkl/commons/Strings.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -86,3 +86,21 @@ fun shlex(input: String): List<String> {
 
   return result
 }
+
+private val springPrefixRegex =
+  Regex(
+    """
+    (?mx)
+    ^
+    [a-z] # starts with lowercase letter
+    [a-z0-9]* # followed by zero or more lowercase letters or digits
+    (?:-[a-z0-9]+)* # followed by possibly kebab-cased
+    (?:\.[a-z][a-z0-9]*(?:-[a-z0-9]+)*)* # followed by dot-separated nested prefixes
+    $
+    """
+      .trimIndent()
+  )
+
+/** Tells if this string is a valid prefix in Spring Boot's `@ConfigurationProperties` annotation */
+val String.isValidConfigurationPropertiesPrefix: Boolean
+  get() = matches(springPrefixRegex)

pkl-commons/src/test/kotlin/org/pkl/commons/StringsTest.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright © 2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.commons
+
+import org.junit.jupiter.api.Test
+
+class StringsTest {
+  @Test
+  fun isValidConfigurationPropertiesPrefix() {
+    val passes = listOf("app.datasource", "my-app.service-config", "app2.v1-api")
+    val negatives =
+      listOf(
+        "myApp.service",
+        "my_app.service",
+        "1app.service",
+        "app..service",
+        ".app.service / app.",
+      )
+    for (value in passes) {
+      assert(value.isValidConfigurationPropertiesPrefix) {
+        "$value should have been valid but was not"
+      }
+    }
+    for (value in negatives) {
+      assert(!value.isValidConfigurationPropertiesPrefix) {
+        "$value should not have been valid but was"
+      }
+    }
+  }
+}