feat: create injectable regex utility function (#536)

This PR introduces a new utility function, `createInjectableRegExp()`,
that converts any regex into an "injectable" version by removing its
leading `^` and trailing `$` anchors.

This is useful when you need to embed an existing pattern, which was
originally designed to match a whole string, inside another regex to
match it as a substring.

Thanks to this, we can replace
[APIFY_PROXY_VALUE_REGEX](https://github.com/apify/apify-shared-js/blob/master/packages/consts/src/regexs.ts#L51)
with
[REGEX_STR_USERNAME_VALUE](https://github.com/apify/apify-proxy/blob/develop/src/proxy_utils.ts#L20)
in Apify proxy and solve this issue
https://github.com/apify/apify-proxy/issues/1079.

The function was created in Apify shared repo, so it can be used in
other places.

Author: lewis-wow

packages/utilities/src/utilities.ts

@@ -429,3 +429,10 @@ export async function timeoutPromise<T>(promise: Promise<T>, timeoutMillis: numb
         timeout = setTimeout(() => callback(new Error(errorMessage)), timeoutMillis) as unknown as number;
     });
 }
+
+/**
+ * Removes the leading ^ and trailing $ from regex
+ */
+export function createInjectableRegExp(regex: RegExp) {
+    return new RegExp(regex.source.replace(/^\^|\$$/g, ''));
+}

test/utilities.test.ts

@@ -398,3 +398,21 @@ describe('BetterSetInterval', () => {
         expect(fn).toHaveBeenCalledTimes(4);
     });
 });
+
+describe('createInjectableRegExp()', () => {
+    it('works with leading ^ and trailing $', () => {
+        expect(utils.createInjectableRegExp(/^[\w._~]+$/)).toStrictEqual(/[\w._~]+/);
+    });
+
+    it('works with leading ^', () => {
+        expect(utils.createInjectableRegExp(/^[\w._~]+/)).toStrictEqual(/[\w._~]+/);
+    });
+
+    it('works with trailing $', () => {
+        expect(utils.createInjectableRegExp(/[\w._~]+$/)).toStrictEqual(/[\w._~]+/);
+    });
+
+    it('works when regex is already injectable', () => {
+        expect(utils.createInjectableRegExp(/[\w._~]+/)).toStrictEqual(/[\w._~]+/);
+    });
+});