How do I validate/limit/filter the input to a GtkEntry? [GTK 2.x]

#include <ctype.h>
#include <gtk/gtk.h>

void insert_text_handler (GtkEntry    *entry,
                          const gchar *text,
                          gint         length,
                          gint        *position,
                          gpointer     data)
{
  GtkEditable *editable = GTK_EDITABLE(entry);
  int i, count=0;
  gchar *result = g_new (gchar, length);

  for (i=0; i < length; i++) {
    if (!isalpha(text[i]))
      continue;
    result[count++] = islower(text[i]) ? toupper(text[i]) : text[i];
  }
  
  if (count > 0) {
    g_signal_handlers_block_by_func (G_OBJECT (editable),
                                     G_CALLBACK (insert_text_handler),
                                     data);
    gtk_editable_insert_text (editable, result, count, position);
    g_signal_handlers_unblock_by_func (G_OBJECT (editable),
                                       G_CALLBACK (insert_text_handler),
                                       data);
  }
  g_signal_stop_emission_by_name (G_OBJECT (editable), "insert_text");

  g_free (result);
}

int main (int   argc,
          char *argv[])
{
  GtkWidget *window;
  GtkWidget *entry;
  
  gtk_init (&argc, &argv);
  
  /* create a new window */
  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  gtk_window_set_title(GTK_WINDOW (window), "GTK Entry");
  g_signal_connect (G_OBJECT (window), "destroy",
                    G_CALLBACK (gtk_main_quit),
                    NULL);

  entry = gtk_entry_new();
  g_signal_connect(G_OBJECT(entry), "insert_text",
		     G_CALLBACK(insert_text_handler),
		     NULL);
  gtk_container_add(GTK_CONTAINER (window), entry);
  gtk_widget_show(entry);
  
  gtk_widget_show(window);
  
  gtk_main();
  return(0);
}