Fix Java version check failing when it can only find a major version

Fixes #52

src/utils/get-java-version.ts

@@ -1,13 +1,16 @@
 import execa = require('execa')
 
-/** Extracts the Java major version from the output of `java -version`. */
+/** Returns the major version of the system's default Java installation. */
 export default async function getJavaVersion() {
   try {
     const { stderr } = await execa('java', ['-version'])
-    const version = stderr.match(/"(\d+\.\d+\.\d+)/)![1]
+    const majorVersionString = stderr.match(JAVA_VERSION_PATTERN)?.groups?.major
+    if (!majorVersionString) {
+      const message = `Could not extract Java major version from "java -version" output!\n${stderr}`
+      throw new Error(message)
+    }
 
-    // `.replace` needed to remove `1.` prefix from versions older than Java 9
-    return parseInt(version.replace(/^1\./, '').match(/^\d+/)![0])
+    return parseInt(majorVersionString)
   } catch (error) {
     if (error.code === 'ENOENT')
       throw new Error(
@@ -18,3 +21,14 @@ export default async function getJavaVersion() {
     throw error
   }
 }
+
+/**
+ * Pattern for extracting the Java major version from the output of
+ * `java -version` (stripping the `1.` prefix from versions prior to Java 9).
+ *
+ * Some example outputs with their respective versions:
+ * - `openjdk version "1.8.0_292"` → `8`
+ * - `openjdk version "11.0.11" 2021-04-20` → `11`
+ * - `java version "15" 2020-09-15` → `15`
+ */
+const JAVA_VERSION_PATTERN = /"(?:1\.)?(?<major>\d+).*?"/