WordPress: Better Internal Link Search modifier to search for posts within a particular category. http://wordpress.org/support/topic/filter-by-categories

gistfile1.php

<?php
/**
 * Search for posts in a category.
 *
 * <code>-categ:{category slug} {query}</code>
 */
function bils_category_search( $results, $args ) {
	$post_types = get_post_types( array( 'public' => true ), 'objects' );
	
	$search_args = array(
		'post_status'    => 'any',
		'post_type'      => array_keys( $post_types ),
		'category_name'  => $args['modifier'][1],
		'paged'          => $args['page'],
		'posts_per_page' => $args['per_page'],
		's'              => $args['s']
	);	

	$posts = get_posts( $search_args );
	if ( $posts ) {
		foreach ( $posts as $post ) {
			$results[] = array(
				'ID'        => $post->ID,
				'title'     => trim( esc_html( strip_tags( get_the_title( $post ) ) ) ),
				'permalink' => get_permalink( $post->ID ),
				'info'      => $post_types[ $post->post_type ]->labels->singular_name,
			);
		}

	}
	
	return $results;
}
add_filter( 'better_internal_link_search_modifier-categ', 'bils_category_search', 10, 2 );

/**
 * Add the help entry for searching posts in a category.
 *
 * Multiple categories can be searched by separating their slugs with a comma.
 */
function bils_categ_search_help( $results ) {
	$results['categ'] = array(
		'title'     => '<strong>-categ:{category slug} {query}</strong></span><span class="item-description">Search your local WordPress installation for posts of that category.</span>',
		'permalink' => home_url( '/' ),
		'info'      => 'Local',
	);
	
	asort( $results );
	
	return $results;
}
add_filter( 'better_internal_link_search_modifier_help', 'bils_categ_search_help' );