proc url-encode {string} {
    variable formMap
    variable alphanumeric

    # The spec says: "non-alphanumeric characters are replaced by '%HH'"
    # 1 leave alphanumerics characters alone
    # 2 Convert every other character to an array lookup
    # 3 Escape constructs that are "special" to the tcl parser
    # 4 "subst" the result, doing all the array substitutions

    regsub -all \[^$alphanumeric\] $string {$formMap(&)} string
    regsub -all {[][{})\\]\)} $string {\\&} string
    return [subst -nocommand $string]
  }