Set & token as numeric on filtered search tokens

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/213154.

This adds the & token to the prefixed-tokens regex/array so the symbol
gets stripped when processed.

Involves a bit of refactoring to improve readability.

app/assets/javascripts/filtered_search/filtered_search_tokenizer.js

@@ -6,7 +6,7 @@ export default class FilteredSearchTokenizer {
     // Values that start with a double quote must end in a double quote (same for single)
 
     const tokenRegex = new RegExp(
-      `(${allowedKeys.join('|')}):(=|!=)?([~%@]?)(?:('[^']*'{0,1})|("[^"]*"{0,1})|(\\S+))`,
+      `(${allowedKeys.join('|')}):(=|!=)?([~%@&]?)(?:('[^']*'{0,1})|("[^"]*"{0,1})|(\\S+))`,
       'g',
     );
     const tokens = [];
@@ -15,17 +15,19 @@ export default class FilteredSearchTokenizer {
     const searchToken =
       input
         .replace(tokenRegex, (match, key, operator, symbol, v1, v2, v3) => {
+          const prefixedTokens = ['~', '%', '@', '&'];
+          const comparisonTokens = ['!=', '='];
           let tokenValue = v1 || v2 || v3;
           let tokenSymbol = symbol;
           let tokenIndex = '';
           let tokenOperator = operator;
 
-          if (tokenValue === '~' || tokenValue === '%' || tokenValue === '@') {
+          if (prefixedTokens.includes(tokenValue)) {
             tokenSymbol = tokenValue;
             tokenValue = '';
           }
 
-          if (tokenValue === '!=' || tokenValue === '=') {
+          if (comparisonTokens.includes(tokenValue)) {
             tokenOperator = tokenValue;
             tokenValue = '';
           }

ee/changelogs/unreleased/213154-re-submitting-an-epic-filter-on-the-issues-search-bar-prepends-.yml

+---
+title: Fix failing filtered search when re-submitting epic tokens
+merge_request: 35205
+author:
+type: fixed

ee/spec/frontend/filtered_search/filtered_search_tokenizer_spec.js

+import IssuableFilteredSearchTokenKeys from 'ee/filtered_search/issuable_filtered_search_token_keys';
+import FilteredSearchTokenizer from '~/filtered_search/filtered_search_tokenizer';
+
+describe('Filtered Search Tokenizer', () => {
+  const allowedKeys = IssuableFilteredSearchTokenKeys.getKeys();
+
+  describe('processTokens', () => {
+    describe('epic tokens', () => {
+      it.each`
+        searchQuery     | operator
+        ${'epic:=&36'}  | ${'='}
+        ${'epic:!=&36'} | ${'!='}
+      `('returns for input containing $searchQuery', ({ searchQuery, operator }) => {
+        const results = FilteredSearchTokenizer.processTokens(searchQuery, allowedKeys);
+
+        expect(results.searchToken).toBe('');
+        expect(results.tokens).toHaveLength(1);
+        expect(results.tokens[0].key).toBe('epic');
+        expect(results.tokens[0].operator).toBe(operator);
+        expect(results.tokens[0].symbol).toBe('&');
+        expect(results.tokens[0].value).toBe('36');
+      });
+
+      it('returns for input containing string values', () => {
+        const results = FilteredSearchTokenizer.processTokens('epic:=any', allowedKeys);
+
+        expect(results.searchToken).toBe('');
+        expect(results.tokens).toHaveLength(1);
+        expect(results.tokens[0].key).toBe('epic');
+        expect(results.tokens[0].operator).toBe('=');
+        expect(results.tokens[0].symbol).toBe('');
+        expect(results.tokens[0].value).toBe('any');
+      });
+    });
+  });
+});