WordPress at a glance
filter is not described

the_category filter-hook . WP 1.2.0

Filters the category or list of categories.

Usage

add_filter( 'the_category', 'filter_function_name_2809', 10, 3 );
function filter_function_name_2809( $thelist, $separator, $parents ){
	// filter...

	return $thelist;
}
$thelist(string)
List of categories for the current post.
$separator(string)
Separator used between the categories.
$parents(string)
How to display the category parents. Accepts 'multiple', 'single', or empty.

Where the hook is called

get_the_category_list()
the_category
Walker_Category_Checklist::start_el()
the_category
wp_popular_terms_checklist()
the_category
wp_link_category_checklist()
the_category
wp-includes/category-template.php 211
return apply_filters( 'the_category', $thelist, $separator, $parents );
wp-admin/includes/class-walker-category-checklist.php 99
esc_html( apply_filters( 'the_category', $category->name, '', '' ) ) . '</div>';
wp-admin/includes/class-walker-category-checklist.php 106
esc_html( apply_filters( 'the_category', $category->name, '', '' ) ) . '</label>';
wp-includes/category-template.php 132
return apply_filters( 'the_category', '', $separator, $parents );
wp-includes/category-template.php 148
return apply_filters( 'the_category', __( 'Uncategorized' ), $separator, $parents );
wp-admin/includes/template.php 217
echo esc_html( apply_filters( 'the_category', $term->name, '', '' ) );
wp-admin/includes/template.php 258
$name = esc_html( apply_filters( 'the_category', $category->name, '', '' ) );
wp-admin/edit-tags.php 550
'<strong>' . apply_filters( 'the_category', get_cat_name( get_option( 'default_category') ), '', '' ) . '</strong>'

Where the hook is used (in WP core)

Does not used.