feat(extensions): Introduce app extensions

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

src/extensions/extra-integrations/youtube/replace-youtube-link.ts

@@ -0,0 +1,34 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { YoutubeMarkdownExtension } from './youtube-markdown-extension'
+import markdownItRegex from 'markdown-it-regex'
+import type MarkdownIt from 'markdown-it'
+import type { RegexOptions } from '../../../external-types/markdown-it-regex/interface'
+
+const protocolRegex = /(?:http(?:s)?:\/\/)?/
+const subdomainRegex = /(?:www.)?/
+const pathRegex = /(?:youtube(?:-nocookie)?\.com\/(?:[^\\/]+\/.+\/|(?:v|e(?:mbed)?)\/|.*[?&]v=)|youtu\.be\/)/
+const idRegex = /([\w-]{11})/
+const tailRegex = /(?:[?&#].*)?/
+const youtubeVideoUrlRegex = new RegExp(
+  `(?:${protocolRegex.source}${subdomainRegex.source}${pathRegex.source}${idRegex.source}${tailRegex.source})`
+)
+const linkRegex = new RegExp(`^${youtubeVideoUrlRegex.source}$`, 'i')
+
+/**
+ * Replacer for youtube links.
+ */
+export const replaceYouTubeLinkMarkdownItPlugin: MarkdownIt.PluginSimple = (markdownIt: MarkdownIt) =>
+  markdownItRegex(markdownIt, {
+    name: 'youtube-link',
+    regex: linkRegex,
+    replace: (match) => {
+      // ESLint wants to collapse this tag, but then the tag won't be valid html anymore.
+      // noinspection CheckTagEmptyBody
+      return `<${YoutubeMarkdownExtension.tagName} id="${match}"></${YoutubeMarkdownExtension.tagName}>`
+    }
+  } as RegexOptions)