Add extensions naming conventions to ext_skel.php

- first character is a letter
- followed by any number of letters, numbers, or underscores
- prefferred are lower case letters

ext/ext_skel.php

@@ -226,6 +226,13 @@ function process_args($argv, $argc) {
 		error('The skeleton directory was not found');
 	}
 
+	// Validate extension name
+	if (!preg_match('/^[a-z][a-z0-9_]+$/i', $options['ext'])) {
+		error('Invalid extension name. Valid names start with a letter,'
+			.' followed by any number of letters, numbers, or underscores.'
+			.' Using only lower case letters is preferred.');
+	}
+
 	$options['ext'] = str_replace(['\\', '/'], '', strtolower($options['ext']));
 
 	return $options;