Allow match replacement functions

.vscode/settings.json

@@ -0,0 +1,3 @@
+{
+    "typescript.tsdk": "node_modules\\typescript\\lib"
+}

package.json

@@ -1,6 +1,6 @@
 {
   "name": "string-replace-middleware",
-  "version": "1.1.0",
+  "version": "1.2.0",
   "description": "Express middleware to replace strings in response stream on the fly",
   "license": "MIT",
   "repository": "bfncs/string-replace-middleware",
@@ -51,4 +51,4 @@
     "singleQuote": true,
     "trailingComma": "es5"
   }
-}
+}

src/index.ts

@@ -1,14 +1,14 @@
 import { NextFunction, Request, Response } from 'express';
 import hijackResponse from 'hijackresponse';
-import stringReplaceStream from './stringReplaceStream';
+import stringReplaceStream, { MatchReplacement } from './stringReplaceStream';
 
-export type Options = Record<'contentTypeFilterRegexp', RegExp>;
+export type ReplaceFunction = (req: Request, res: Response) => MatchReplacement;
 
-export type ReplaceFunction = (req: Request, res: Response) => string;
-
-const defaultOptions: Options = {
+const defaultOptions = {
   contentTypeFilterRegexp: /^text\/|^application\/json$|^application\/xml$/,
+  useRegExp: false,
 };
+type Options = typeof defaultOptions;
 
 export const stringReplace = (
   replacements: Record<string, string | ReplaceFunction>,
@@ -39,7 +39,7 @@ export const stringReplace = (
         }
         res.removeHeader('content-length');
 
-        let scopedReplacements: Record<string, string>;
+        let scopedReplacements: Record<string, MatchReplacement>;
         if (hasFunctionReplacements) {
           // If we have dynamic replacements, calculate for this request
           scopedReplacements = { ...stringReplacements };
@@ -51,7 +51,13 @@ export const stringReplace = (
           scopedReplacements = stringReplacements;
         }
 
-        res.pipe(stringReplaceStream(scopedReplacements)).pipe(res);
+        res
+          .pipe(
+            stringReplaceStream(scopedReplacements, {
+              useRegExp: options.useRegExp,
+            })
+          )
+          .pipe(res);
       } else {
         return res.unhijack();
       }

src/stringReplaceStream.ts

@@ -1,44 +1,53 @@
 import { Transform, TransformCallback } from 'stream';
 import escapeStringRegexp from 'escape-string-regexp';
 
-type Options = {
-  encoding: BufferEncoding;
-  ignoreCase: boolean;
-};
+/** The replacer string or function passed to string.replace(text: string, replacer: *MatchReplacement*) */
+export type MatchReplacement =
+  | string
+  | ((matchedSubstring: string, ...capturedGroups: string[]) => string);
+
 type Replacer = {
   matcher: RegExp;
-  replace: string;
+  replace: MatchReplacement;
 };
 
+type Options = {
+  encoding: BufferEncoding;
+  ignoreCase: boolean;
+  useRegExp: boolean;
+};
 const defaultOptions: Options = {
   encoding: 'utf8',
   ignoreCase: true,
+  useRegExp: false,
 };
 
 function buildReplacers(
-  replacements: Record<string, string>,
+  replacements: Record<string, MatchReplacement>,
   opts: Options
 ): Replacer[] {
   return Object.keys(replacements)
     .sort((a, b) => b.length - a.length)
     .map(search => ({
       matcher: new RegExp(
-        escapeStringRegexp(search),
+        opts.useRegExp ? search : escapeStringRegexp(search),
         opts.ignoreCase ? 'gmi' : 'gm'
       ),
       replace: replacements[search],
     }));
 }
 
-function getMaxSearchLength(replacements: Record<string, string>): number {
+function getMaxSearchLength(
+  replacements: Record<string, MatchReplacement>
+): number {
   return Object.keys(replacements).reduce(
     (acc, search) => Math.max(acc, search.length),
     0
   );
 }
 
 export default function StringReplaceStream(
-  replacements: Record<string, string>,
+  replacements: Record<string, MatchReplacement>,
   options: Partial<Options> = {}
 ) {
   const opts: Options = { ...defaultOptions, ...options };
@@ -65,7 +74,7 @@ export default function StringReplaceStream(
       body =
         body
           .slice(0, replaceBefore)
-          .replace(replacer.matcher, replacer.replace) +
+          .replace(replacer.matcher, replacer.replace as string) +
         body.slice(replaceBefore);
     });
 
@@ -98,7 +107,8 @@ export default function StringReplaceStream(
     }
 
     const body = replacers.reduce(
-      (acc, replacer) => acc.replace(replacer.matcher, replacer.replace),
+      (acc, replacer) =>
+        acc.replace(replacer.matcher, replacer.replace as string),
       tail
     );
     cb(null, body);