Allow underscores in plugin/theme slugs for test scaffolding

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

Author: Copilot

features/scaffold-block.feature

@@ -31,7 +31,7 @@ Feature: WordPress block code scaffolding
       """
 
   Scenario: Scaffold a block for an invalid plugin slug
-    When I try `wp scaffold plugin plugin.name.with.dots`
+    When I run `wp scaffold plugin plugin.name.with.dots`
     And I try `wp scaffold block some-block --plugin=plugin.name.with.dots`
     Then STDERR should contain:
       """

features/scaffold-plugin-tests.feature

@@ -243,21 +243,14 @@ Feature: Scaffold plugin unit tests
     When I try `wp scaffold plugin-tests my-plugin/`
     Then STDERR should be:
       """
-      Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters and dashes.
+      Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters, underscores, and dashes.
       """
     And the return code should be 1
 
     When I try `wp scaffold plugin-tests my-plugin\\`
     Then STDERR should be:
       """
-      Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters and dashes.
-      """
-    And the return code should be 1
-
-    When I try `wp scaffold plugin-tests my_plugin`
-    Then STDERR should be:
-      """
-      Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters and dashes.
+      Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters, underscores, and dashes.
       """
     And the return code should be 1
 

features/scaffold-theme-tests.feature

@@ -222,21 +222,14 @@ Feature: Scaffold theme unit tests
     When I try `wp scaffold theme-tests t12child/`
     Then STDERR should be:
       """
-      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters and dashes.
+      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters, underscores, and dashes.
       """
     And the return code should be 1
 
     When I try `wp scaffold theme-tests t12child\\`
     Then STDERR should be:
       """
-      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters and dashes.
-      """
-    And the return code should be 1
-
-    When I try `wp scaffold theme-tests t12_child`
-    Then STDERR should be:
-      """
-      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters and dashes.
+      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters, underscores, and dashes.
       """
     And the return code should be 1
 

src/Scaffold_Command.php

@@ -829,9 +829,9 @@ private function scaffold_plugin_theme_tests( $args, $assoc_args, $type ) {
 			if ( in_array( $slug, [ '.', '..' ], true ) ) {
 				WP_CLI::error( "Invalid {$type} slug specified. The slug cannot be '.' or '..'." );
 			}
-			// Validate slug contains only alphanumeric characters and dashes.
-			if ( ! preg_match( '/^[a-zA-Z0-9-]+$/', $slug ) ) {
-				WP_CLI::error( "Invalid {$type} slug specified. The slug can only contain alphanumeric characters and dashes." );
+			// Validate slug contains only alphanumeric characters, underscores, and dashes.
+			if ( ! preg_match( '/^[a-zA-Z0-9_-]+$/', $slug ) ) {
+				WP_CLI::error( "Invalid {$type} slug specified. The slug can only contain alphanumeric characters, underscores, and dashes." );
 			}
 			if ( 'theme' === $type ) {
 				$theme = wp_get_theme( $slug );