SASS

gistfile1.txt

$col-numbers: "1", "2", "3", "4", "5", "6",
              "7", "8", "9", "10", "11", "12";
$selector: ();

@each $item in $col-numbers {
  $selector: append($selector, unquote('.col-md-#{$item}'), comma);
}

#{$selector} {
  padding-right: 0px;
  padding-left: 0px;
}

#=>

.col-md-1, .col-md-2, .col-md-3, .col-md-4, .col-md-5, .col-md-6, .col-md-7, .col-md-8, .col-md-9, .col-md-10, .col-md-11, .col-md-12 {
  padding-right: 0px;
  padding-left: 0px; }