exists() does not consider URL a file or folder. add is_url()

Matlab R2024b allows URLs to be treated like files and folders.
For our stdlib, we give the option (default false) to not consider
URLs as files and folders.

Author: scivision

+stdlib/exists.m

@@ -9,12 +9,16 @@
 %%% Outputs
 % * ok: true if exists
 
-function ok = exists(p)
+function y = exists(p)
 arguments
   p (1,1) string
 end
 
-ok = isfile(p) || isfolder(p);
+% Matlab >= R2024b allowed URLs to act like files or folders.
+% fileattrib() does not consider URLs to be a file or folder
+% at least through Matlab R2025a.
+
+y = stdlib.len(p) && fileattrib(p) == 1;
 
 end
 

+stdlib/is_url.m

@@ -0,0 +1,26 @@
+function y = is_url(s)
+arguments
+  s (1,1) string
+end
+
+try
+  y = startsWith(s, alphanumericsPattern + "://");
+catch e
+  % Matlab < R2020b
+  if ~strcmp(e.identifier, "MATLAB:UndefinedFunction") && ...
+    ~strcmp(e.identifier, "Octave:undefined-function")
+      rethrow(e)
+  end
+
+  % https://www.mathworks.com/help/matlab/import_export/work-with-remote-data.html
+
+  y = startsWith(s, "http://") || startsWith(s, "https://") || ...
+      startsWith(s, "ftp://") || startsWith(s, "file://") || ...
+      startsWith(s, "s3://") || startsWith(s, "hdfs://") || ...
+      startsWith(s, "wasbs://");
+end
+
+end
+
+%!assert (is_url("http://example.com"), true)
+%!assert (is_url("//server"), false)