refactor: move placeholder regexp to another PR

src/index.js

@@ -16,7 +16,6 @@ const pluginName = 'mini-css-extract-plugin';
 const REGEXP_CHUNKHASH = /\[chunkhash(?::(\d+))?\]/i;
 const REGEXP_CONTENTHASH = /\[contenthash(?::(\d+))?\]/i;
 const REGEXP_NAME = /\[name\]/i;
-const REGEXP_PLACEHOLDERS = /\[(name|id|chunkhash)\]/g;
 const DEFAULT_FILENAME = '[name].css';
 
 class CssDependency extends webpack.Dependency {
@@ -131,9 +130,12 @@ class MiniCssExtractPlugin {
 
     if (!this.options.chunkFilename) {
       const { filename } = this.options;
+      const hasName = filename.includes('[name]');
+      const hasId = filename.includes('[id]');
+      const hasChunkHash = filename.includes('[chunkhash]');
 
       // Anything changing depending on chunk is fine
-      if (REGEXP_PLACEHOLDERS.test(filename)) {
+      if (hasChunkHash || hasName || hasId) {
         this.options.chunkFilename = filename;
       } else {
         // Elsewise prefix '[id].' in front of the basename to make it changing