refactor(dev-infra): extract the commit message parsing function into its own file (#38429)

Extracts the commit message parsing function into its own file.

PR Close #38429

dev-infra/commit-message/parse.ts

@@ -0,0 +1,73 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+/** A parsed commit message. */
+export interface ParsedCommitMessage {
+  header: string;
+  body: string;
+  bodyWithoutLinking: string;
+  type: string;
+  scope: string;
+  subject: string;
+  isFixup: boolean;
+  isSquash: boolean;
+  isRevert: boolean;
+}
+
+/** Regex determining if a commit is a fixup. */
+const FIXUP_PREFIX_RE = /^fixup! /i;
+/** Regex finding all github keyword links. */
+const GITHUB_LINKING_RE = /((closed?s?)|(fix(es)?(ed)?)|(resolved?s?))\s\#(\d+)/ig;
+/** Regex determining if a commit is a squash. */
+const SQUASH_PREFIX_RE = /^squash! /i;
+/** Regex determining if a commit is a revert. */
+const REVERT_PREFIX_RE = /^revert:? /i;
+/** Regex determining the scope of a commit if provided. */
+const TYPE_SCOPE_RE = /^(\w+)(?:\(([^)]+)\))?\:\s(.+)$/;
+/** Regex determining the entire header line of the commit. */
+const COMMIT_HEADER_RE = /^(.*)/i;
+/** Regex determining the body of the commit. */
+const COMMIT_BODY_RE = /^.*\n\n([\s\S]*)$/;
+
+/** Parse a full commit message into its composite parts. */
+export function parseCommitMessage(commitMsg: string): ParsedCommitMessage {
+  let header = '';
+  let body = '';
+  let bodyWithoutLinking = '';
+  let type = '';
+  let scope = '';
+  let subject = '';
+
+  if (COMMIT_HEADER_RE.test(commitMsg)) {
+    header = COMMIT_HEADER_RE.exec(commitMsg)![1]
+                 .replace(FIXUP_PREFIX_RE, '')
+                 .replace(SQUASH_PREFIX_RE, '');
+  }
+  if (COMMIT_BODY_RE.test(commitMsg)) {
+    body = COMMIT_BODY_RE.exec(commitMsg)![1];
+    bodyWithoutLinking = body.replace(GITHUB_LINKING_RE, '');
+  }
+
+  if (TYPE_SCOPE_RE.test(header)) {
+    const parsedCommitHeader = TYPE_SCOPE_RE.exec(header)!;
+    type = parsedCommitHeader[1];
+    scope = parsedCommitHeader[2];
+    subject = parsedCommitHeader[3];
+  }
+  return {
+    header,
+    body,
+    bodyWithoutLinking,
+    type,
+    scope,
+    subject,
+    isFixup: FIXUP_PREFIX_RE.test(commitMsg),
+    isSquash: SQUASH_PREFIX_RE.test(commitMsg),
+    isRevert: REVERT_PREFIX_RE.test(commitMsg),
+  };
+}