Allow prerelease versions in release headers (#130)

Currently, this tool expects the version in a release header to be a
version that looks like `<major>.<minor>.<patch>` and rejects other
formats. However, it may be useful to publish a prerelease version of a
package so that integration with other products/services can be tested
in a more "live" manner, particularly if that package is a part of a
monorepo. A prerelease version looks like
`<major>.<minor>.<patch>-<prerelease identifier>.<number>`, where
`<prerelease identifier>` can be an arbitrary word like `alpha`, `beta`,
or `rc`. This commit allows such versions.

src/parse-changelog.test.ts

@@ -199,6 +199,45 @@ describe('parseChangelog', () => {
     expect(changelog.getUnreleasedChanges()).toStrictEqual({});
   });
 
+  it('should parse changelog with prereleases', () => {
+    const changelog = parseChangelog({
+      changelogContent: outdent`
+        # Changelog
+        All notable changes to this project will be documented in this file.
+
+        The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+        and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+        ## [Unreleased]
+
+        ## [1.0.0-rc.1] - 2020-01-01
+        ### Changed
+        - Something else
+
+        ## [0.0.2-beta.1] - 2020-01-01
+        ### Fixed
+        - Something
+
+        ## [0.0.1-alpha.1] - 2020-01-01
+        ### Changed
+        - Something
+
+        [Unreleased]: https://github.com/ExampleUsernameOrOrganization/ExampleRepository/compare/v1.0.0-rc.1...HEAD
+        [1.0.0-rc.1]: https://github.com/ExampleUsernameOrOrganization/ExampleRepository/compare/v0.0.2-beta.1...v1.0.0-rc.1
+        [0.0.2-beta.1]: https://github.com/ExampleUsernameOrOrganization/ExampleRepository/compare/v0.0.1-alpha.1...v0.0.2-beta.1
+        [0.0.1-alpha.1]: https://github.com/ExampleUsernameOrOrganization/ExampleRepository/releases/tag/v0.0.1-alpha.1
+        `,
+      repoUrl:
+        'https://github.com/ExampleUsernameOrOrganization/ExampleRepository',
+    });
+
+    expect(changelog.getReleases()).toStrictEqual([
+      { date: '2020-01-01', status: undefined, version: '1.0.0-rc.1' },
+      { date: '2020-01-01', status: undefined, version: '0.0.2-beta.1' },
+      { date: '2020-01-01', status: undefined, version: '0.0.1-alpha.1' },
+    ]);
+  });
+
   it('should parse changelog with release statuses', () => {
     const changelog = parseChangelog({
       changelogContent: outdent`
@@ -508,6 +547,32 @@ describe('parseChangelog', () => {
     ).toThrow(`Malformed release header: '## [1.0.0 - 2020-01-01'`);
   });
 
+  it('should throw if version in release header is not SemVer-compatible', () => {
+    const brokenChangelog = outdent`
+      # Changelog
+      All notable changes to this project will be documented in this file.
+
+      The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+      and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+      ## [Unreleased]
+
+      ## [1.2.3.4]
+      ### Changed
+      - Something else
+
+      [Unreleased]: https://github.com/ExampleUsernameOrOrganization/ExampleRepository/compare/v1.2.3.4...HEAD
+      [1.2.3.4]: https://github.com/ExampleUsernameOrOrganization/ExampleRepository/releases/tag/v1.2.3.4
+      `;
+    expect(() =>
+      parseChangelog({
+        changelogContent: brokenChangelog,
+        repoUrl:
+          'https://github.com/ExampleUsernameOrOrganization/ExampleRepository',
+      }),
+    ).toThrow(`Invalid SemVer version in release header: '## [1.2.3.4]`);
+  });
+
   it('should throw if release header uses the wrong header level', () => {
     const brokenChangelog = outdent`
       # Changelog

src/parse-changelog.ts

@@ -1,3 +1,4 @@
+import semver from 'semver';
 import Changelog from './changelog';
 import { ChangeCategory, unreleased } from './constants';
 
@@ -107,12 +108,18 @@ export function parseChangelog({
   for (const line of contentfulChangelogLines) {
     if (line.startsWith('## [')) {
       const results = line.match(
-        /^## \[(\d+\.\d+\.\d+)\](?: - (\d\d\d\d-\d\d-\d\d))?(?: \[(\w+)\])?/u,
+        /^## \[([^[\]]+)\](?: - (\d\d\d\d-\d\d-\d\d))?(?: \[(\w+)\])?/u,
       );
       if (results === null) {
         throw new Error(`Malformed release header: '${truncated(line)}'`);
       }
 
+      if (semver.valid(results[1]) === null) {
+        throw new Error(
+          `Invalid SemVer version in release header: '${truncated(line)}'`,
+        );
+      }
+
       // Trailing newline removed because the release section is expected to
       // be prefixed by a newline.
       finalizePreviousChange({