Source

File: wp-includes/formatting.php

function sanitize_user( $username, $strict = false ) {
	$raw_username = $username;
	$username     = wp_strip_all_tags( $username );
	$username     = remove_accents( $username );
	// Kill octets.
	$username = preg_replace( '|%([a-fA-F0-9][a-fA-F0-9])|', '', $username );
	// Kill entities.
	$username = preg_replace( '/&.+?;/', '', $username );

	// If strict, reduce to ASCII for max portability.
	if ( $strict ) {
		$username = preg_replace( '|[^a-z0-9 _.\-@]|i', '', $username );
	}

	$username = trim( $username );
	// Consolidate contiguous whitespace.
	$username = preg_replace( '|\s+|', ' ', $username );

	/**
	 * Filters a sanitized username string.
	 *
	 * @since 2.0.1
	 *
	 * @param string $username     Sanitized username.
	 * @param string $raw_username The username prior to sanitization.
	 * @param bool   $strict       Whether to limit the sanitization to specific characters.
	 */
	return apply_filters( 'sanitize_user', $username, $raw_username, $strict );
}