bp_activity_at_name_filter( string $content, int $activity_id )

function bp_activity_at_name_filter( $content, $activity_id = 0 ) {

	// Are mentions disabled?
	if ( ! bp_activity_do_mentions() ) {
		return $content;
	}

	// Try to find mentions.
	$usernames = bp_activity_find_mentions( $content );

	// No mentions? Stop now!
	if ( empty( $usernames ) )
		return $content;

	// We don't want to link @mentions that are inside of links, so we
	// temporarily remove them.
	$replace_count = 0;
	$replacements = array();
	foreach ( $usernames as $username ) {
		// Prevent @ name linking inside <a> tags.
		preg_match_all( '/(<a.*?(?!<\/a>)@' . $username . '.*?<\/a>)/', $content, $content_matches );
		if ( ! empty( $content_matches[1] ) ) {
			foreach ( $content_matches[1] as $replacement ) {
				$replacements[ '#BPAN' . $replace_count ] = $replacement;
				$content = str_replace( $replacement, '#BPAN' . $replace_count, $content );
				$replace_count++;
			}
		}
	}

	// Linkify the mentions with the username.
	foreach ( (array) $usernames as $user_id => $username ) {
		$content = preg_replace( '/(@' . $username . '\b)/', "<a class='bp-suggestions-mention' href='" . bp_core_get_user_domain( $user_id ) . "' rel='nofollow'>@$username</a>", $content );
	}

	// Put everything back.
	if ( ! empty( $replacements ) ) {
		foreach ( $replacements as $placeholder => $original ) {
			$content = str_replace( $placeholder, $original, $content );
		}
	}

	// Return the content.
	return $content;
}