Pick up img tag title attribute from custom field values

rlv_img_alt.php

<?php

// Add this to theme functions.php.
add_filter( 'relevanssi_custom_field_value', 'rlv_img_alt', 10 );
function rlv_img_alt( $value ) {
	if ( ! is_array( $value ) ) {
		$value = array( $value );
	}
	$new_values = array();
	foreach ( $value as $value_string ) {
		if ( '<img' === substr( $value_string, 0, 4 ) ) {
			$m = preg_match( '/title=[\'"](.*?)[\'"]/', $value_string, $matches );
			if ( $m ) {
				$value_string = $matches[1];
				$value_string = str_replace( array( 'listing', 'logo', '_' ), ' ', $value_string );
			}
		}
		$new_values[] = $value_string;
	}
	return $new_values;
}