Page name searches are case-sensitive. If $wgCapitalLinks is true (this is the default), the first letter is case-insensitive. If the TitleKey extension is enabled, page name searches are entirely case-insensitive.
The opensearch API module handles formats differently. Only XML and JSON (and their formatted versions) are recognized, every other valid format parameter results in JSON. The JSON format returns an array containing the search query and an array of result titles: JSON
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<?xmlversion="1.0"?><SearchSuggestionversion="2.0"xmlns="http://opensearch.org/searchsuggest2"><Queryxml:space="preserve">api</Query><Section><Item><Textxml:space="preserve">Application programming interface</Text><Descriptionxml:space="preserve">An application programming interface (API) is a particular set of rules and specifications that a software program can follow to access and make use of the services and resources provided by another particular software program that implements that API. </Description><Urlxml:space="preserve">http://en.wikipedia.org/wiki/Application_programming_interface</Url></Item><Item><Textxml:space="preserve">Apiales</Text><Descriptionxml:space="preserve">The Apiales are an order of flowering plants. </Description><Urlxml:space="preserve">http://en.wikipedia.org/wiki/Apiales</Url><Imagesource="http://upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Umbella.jpg/50px-Umbella.jpg"width="50"height="46"/></Item></Section></SearchSuggestion>
The above content has been placed in a collapsed box for improved usability.