diff --git a/wp-admin/includes/widgets.php b/wp-admin/includes/widgets.php
index ac98cdb527..af33787fa6 100644
--- a/wp-admin/includes/widgets.php
+++ b/wp-admin/includes/widgets.php
@@ -162,7 +162,7 @@ function next_widget_id_number( $id_base ) {
 	$number = 1;
 
 	foreach ( $wp_registered_widgets as $widget_id => $widget ) {
-		if ( preg_match( '/' . $id_base . '-([0-9]+)$/', $widget_id, $matches ) ) {
+		if ( preg_match( '/' . preg_quote( $id_base, '/' ) . '-([0-9]+)$/', $widget_id, $matches ) ) {
 			$number = max( $number, $matches[1] );
 		}
 	}
diff --git a/wp-includes/class-wp-widget.php b/wp-includes/class-wp-widget.php
index b177c82053..71381d35e6 100644
--- a/wp-includes/class-wp-widget.php
+++ b/wp-includes/class-wp-widget.php
@@ -164,7 +164,6 @@ class WP_Widget {
 			$id_base = strtolower( $id_base );
 		} else {
 			$id_base = preg_replace( '/(wp_)?widget_/', '', strtolower( get_class( $this ) ) );
-			$id_base = str_replace( '\\', '-', $id_base );
 		}
 
 		$this->id_base         = $id_base;
@@ -173,7 +172,7 @@ class WP_Widget {
 		$this->widget_options  = wp_parse_args(
 			$widget_options,
 			array(
-				'classname'                   => $this->option_name,
+				'classname'                   => str_replace( '\\', '_', $this->option_name ),
 				'customize_selective_refresh' => false,
 			)
 		);
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 12070c10e3..d9c1bc9558 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-50960';
+$wp_version = '5.8-alpha-50961';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
diff --git a/wp-includes/widgets.php b/wp-includes/widgets.php
index 1987023463..6030ecc9fc 100644
--- a/wp-includes/widgets.php
+++ b/wp-includes/widgets.php
@@ -753,8 +753,13 @@ function dynamic_sidebar( $index = 1 ) {
 				$classname_ .= '_' . get_class( $cn );
 			}
 		}
-		$classname_                 = ltrim( $classname_, '_' );
-		$params[0]['before_widget'] = sprintf( $params[0]['before_widget'], $id, $classname_ );
+		$classname_ = ltrim( $classname_, '_' );
+
+		$params[0]['before_widget'] = sprintf(
+			$params[0]['before_widget'],
+			str_replace( '\\', '_', $id ),
+			$classname_
+		);
 
 		/**
 		 * Filters the parameters passed to a widget's display callback.