Only search the PMPro Member Directory for matching usernames or display names.

my_pmpro_member_directory_sql_search_where_username_display_name_only.php

<?php
/**
 * Only search the PMPro Member Directory for matching usernames or display names.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_member_directory_sql_search_where_username_display_name_only( $sql_search_where, $s ) {
	// Limit search to username and display name only.
	$sql_search_where = "u.user_login LIKE '%" . esc_sql( $s ) . "%' OR u.display_name LIKE '%" . esc_sql( $s ) . "%'";

	// Alternate query, limit search for username, email, and display name.
	//$sql_search_where = "u.user_login LIKE '%" . esc_sql( $s ) . "%' OR u.user_email LIKE '%" . esc_sql( $s ) . "%' OR u.display_name LIKE '%" . esc_sql( $s ) . "%'";

	// Build a new query
	$sql_search_where = ' AND ( ' . $sql_search_where . ' ) ';

	return $sql_search_where;
}
add_filter( 'pmpro_member_directory_sql_search_where', 'my_pmpro_member_directory_sql_search_where_username_display_name_only', 10, 2 );