docs(readme): explain named regex groups in replacementPatterns

README.md

@@ -200,7 +200,7 @@ Options:
 `config.json`:
 
 * `ignorePatterns`: An array of objects holding regular expressions which a link is checked against and skipped for checking in case of a match.
-* `replacementPatterns`: An array of objects holding regular expressions which are replaced in a link with their corresponding replacement string. This behavior allows (for example) to adapt to certain platform conventions hosting the Markdown. The special replacement `{{BASEURL}}` can be used to dynamically link to the current working directory (for example that `/` points to the root of your current working directory).
+* `replacementPatterns`: An array of objects holding regular expressions which are replaced in a link with their corresponding replacement string. This behavior allows (for example) to adapt to certain platform conventions hosting the Markdown. The special replacement `{{BASEURL}}` can be used to dynamically link to the current working directory (for example that `/` points to the root of your current working directory). This parameter supports named regex groups the same way as `string.replace` [method](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace#specifying_a_string_as_the_replacement) in node.
 * `httpHeaders`: The headers are only applied to links where the link **starts with** one of the supplied URLs in the `urls` section.
 * `timeout` timeout in [zeit/ms](https://www.npmjs.com/package/ms) format. (e.g. `"2000ms"`, `20s`, `1m`). Default `10s`.
 * `retryOn429` if this is `true` then retry request when response is an HTTP code 429 after the duration indicated by `retry-after` header.
@@ -232,6 +232,10 @@ Options:
       "pattern": "%20",
       "replacement": "-",
       "global": true
+    },
+    {
+      "pattern": "images/(?<filename>.*)",
+      "replacement": "assets/$<filename>"
     }
   ],
   "httpHeaders": [

test/markdown-link-check.test.js

@@ -370,6 +370,33 @@ describe('markdown-link-check', function () {
             done();
         });
     });
+
+    it('should correctly resolve regex named groups in replacement patterns', function (done) {
+        markdownLinkCheck(fs.readFileSync(path.join(dirname, 'regex-groups-replacement.md')).toString().replace(/%%BASE_URL%%/g, 'file://' + dirname), {baseUrl: 'file://' + dirname, projectBaseUrl: 'file://' + dirname + "/..",replacementPatterns: [
+            {pattern: '^/', replacement: "{{BASEURL}}/"},
+            {pattern: 'folder-to-be-ignored/(?<filename>.*)', replacement: '$<filename>'}
+        ]}, function (err, results) {
+            expect(err).to.be(null);
+            expect(results).to.be.an('array');
+
+            const expected = [
+                { statusCode: 200, status: 'alive' },
+                { statusCode: 200, status: 'alive' },
+                { statusCode: 200, status: 'alive' },
+                { statusCode: 200, status: 'alive' }
+            ];
+
+            expect(results.length).to.be(expected.length);
+
+            for (let i = 0; i < results.length; i++) {
+                expect(results[i].statusCode).to.be(expected[i].statusCode);
+                expect(results[i].status).to.be(expected[i].status);
+            }
+
+            done();
+        });
+    });
+
     it('check hash links', function (done) {
         markdownLinkCheck(fs.readFileSync(path.join(dirname, 'hash-links.md')).toString(), {}, function (err, result) {
             expect(err).to.be(null);

test/regex-groups-replacement.md

@@ -0,0 +1,9 @@
+# Sample
+
+This is a test file leveraging special replacement patterns:
+
+![img](hello.jpg) (alive)
+![txt](file.md) (alive)
+
+![img](folder-to-be-ignored/hello.jpg) (replaced and then alive)
+![txt](folder-to-be-ignored/file.md) (replaced and then alive)