Fixes errors in GFM slash commands matcher and add tests for default matcher

See !8975

app/assets/javascripts/gfm_auto_complete.js.es6

@@ -83,12 +83,12 @@
         _a = decodeURI("%C3%80");
         _y = decodeURI("%C3%BF");
 
-        regexp = new RegExp("^(?:\\B|[^a-zA-Z0-9_" + atSymbolsWithoutBar + "]|\\s)" + flag + "(?![" + atSymbolsWithBar + "])(([A-Za-z" + _a + "-" + _y + "0-9_\'\.\+\-]|[^\\x00-\\x7a])*)$", 'gi');
+        regexp = new RegExp("^(?:\\B|[^a-zA-Z0-9_" + atSymbolsWithoutBar + "]|\\s)" + flag + "(?!" + atSymbolsWithBar + ")((?:[A-Za-z" + _a + "-" + _y + "0-9_\'\.\+\-]|[^\\x00-\\x7a])*)$", 'gi');
 
         match = regexp.exec(subtext);
 
         if (match) {
-          return (match[1] || match[1] === "") ? match[1] : match[2];
+          return match[1];
         } else {
           return null;
         }

changelogs/unreleased/gfm-autocomplete-fixes.yml

+---
+title: Fix errors in slash commands matcher, add simple test coverage
+merge_request:
+author: YarNayar

spec/javascripts/gfm_auto_complete_spec.js.es6

+/* eslint no-param-reassign: "off" */
+
 require('~/gfm_auto_complete');
 require('vendor/jquery.caret');
 require('vendor/jquery.atwho');
@@ -63,6 +65,61 @@ describe('GfmAutoComplete', function () {
     });
   });
 
+  describe('DefaultOptions.matcher', function () {
+    const defaultMatcher = (context, flag, subtext) => (
+      GfmAutoComplete.DefaultOptions.matcher.call(context, flag, subtext)
+    );
+
+    const flagsUseDefaultMatcher = ['@', '#', '!', '~', '%'];
+    const otherFlags = ['/', ':'];
+    const flags = flagsUseDefaultMatcher.concat(otherFlags);
+
+    const flagsHash = flags.reduce((hash, el) => { hash[el] = null; return hash; }, {});
+    const atwhoInstance = { setting: {}, app: { controllers: flagsHash } };
+
+    const minLen = 1;
+    const maxLen = 20;
+    const argumentSize = [minLen, maxLen / 2, maxLen];
+
+    const allowedSymbols = ['', 'a', 'n', 'z', 'A', 'Z', 'N', '0', '5', '9', 'А', 'а', 'Я', 'я', '.', '\'', '+', '-', '_'];
+    const jointAllowedSymbols = allowedSymbols.join('');
+
+    describe('should match regular symbols', () => {
+      flagsUseDefaultMatcher.forEach((flag) => {
+        allowedSymbols.forEach((symbol) => {
+          argumentSize.forEach((size) => {
+            const query = new Array(size + 1).join(symbol);
+            const subtext = flag + query;
+
+            it(`matches argument "${flag}" with query "${subtext}"`, () => {
+              expect(defaultMatcher(atwhoInstance, flag, subtext)).toBe(query);
+            });
+          });
+        });
+
+        it(`matches combination of allowed symbols for flag "${flag}"`, () => {
+          const subtext = flag + jointAllowedSymbols;
+
+          expect(defaultMatcher(atwhoInstance, flag, subtext)).toBe(jointAllowedSymbols);
+        });
+      });
+    });
+
+    describe('should not match special sequences', () => {
+      const ShouldNotBeFollowedBy = flags.concat(['\x00', '\x10', '\x3f', '\n', ' ']);
+
+      flagsUseDefaultMatcher.forEach((atSign) => {
+        ShouldNotBeFollowedBy.forEach((followedSymbol) => {
+          const seq = atSign + followedSymbol;
+
+          it(`should not match "${seq}"`, () => {
+            expect(defaultMatcher(atwhoInstance, atSign, seq)).toBe(null);
+          });
+        });
+      });
+    });
+  });
+
   describe('isLoading', function () {
     it('should be true with loading data object item', function () {
       expect(GfmAutoComplete.isLoading({ name: 'loading' })).toBe(true);