How to search in a Custom Field?

I have a custom post type called LAW.
LAW has three custom fields: LAW_DATE, LAW_TEXT and LAW_AUTHOR.
I want to build a search page that allows me to search independently each of these custom fields.
I.e. different criteria for each field, combined with and/or to each other.

Thanks.

The following function needs to be put in the functions.php of your template code. Or in a plugin.

function custom_search_query( $request ) {
    $query = new WP_Query();  // the query isn't run if we don't pass any query vars
    $query->parse_query($request);

    $request['post_type'] = 'LAW';

    // this is the actual manipulation; do whatever you need here
    if(isset($_GET['search']))
        $options = $_GET['search'];
    if (!empty($options)) {
        $i = 0;
        $request['meta_query'] = array(); // resetting any previously selected meta_queries that might "linger" and cause weird behaviour.
        // CAREFUL HERE ^ might not be desired behaviour

        foreach($options AS $key => $value) {
            $request['meta_query'][$i]['key'] = $key;
            $request['meta_query'][$i]['value'] = array($value);
            $request['meta_query'][$i]['compare'] = 'IN';
            $request['meta_query'][$i]['type'] = 'CHAR';
            $i++;
        }
    }

    return($request);
}
add_filter( 'request', 'custom_search_query' );

The above function assumes that the HTML form will be similar to this:

<input type="text|number" name="search[keyname1]" value="value1" />
<input type="text|number" name="search[keyname2]" value="value2" />

It makes no validation on the user input, (wordpress might do some of that but it’s better if you do it).