Besides Wikipedia internal search engine, there are various search engines that can provide domain-specific searches, which let you search Wikipedia specifically. Searches are based on the text as shown by the browser, so wiki markup is irrelevant. Depending on your browser, you may also be able to use tools that allow you to search Wikipedia using bookmarklets.
In general, external search engines are faster than a Wikipedia search. However, because the search engine's cache is based on when the site was indexed, the search may not return newly created pages. Similarly, the search engine's cached version of the page will not be as up-to-date as the link to Wikipedia itself. Also, when returning Wikipedia articles in a regular search, mirrors and forks of Wikipedia content frequently rank higher than the actual Wikipedia articles because of search engine optimization techniques.
These issues may be less of a problem when using certain search engines that process Wikipedia differently: