Sass に文字列置換の関数を追加し、更に引数で関数を指定できる様にしてみる

sample.scss

$tmp: "";
.sample:after {
  $tmp: str-replace("/_./", "|x|x.upcase.sub!('_', '')", "test_hoge");
  content: "#{$tmp}";
}
// 結果：content: "testHoge";

$tmp: "";
.sample2:after{
  $tmp: str-replace("_", "", "test_hoge");
  content: "#{$tmp}";
}
// 結果：content: "testhoge";

str_replace.rb

# 参考：http://www.skyward-design.net/blog/archives/000128.html
require "sass"

module WakuworksFunctions
  def str_replace(search_cond, replace_str, str)
    assert_type search_cond, :String
    assert_type replace_str, :String
    assert_type str, :String

    begin
      replace = eval "lambda { " + replace_str.value + " }"
    rescue
      replace = replace_str.value
    end

    is_proc = replace.class == Proc

    begin
      search = eval search_cond.value
    rescue
      search = search_cond.value
    end

    if (search_cond.class == Regexp) then
      val = is_proc ?
        str.value.gsub(search, &replace) :
        str.value.gsub(search, replace)
    else
      val = is_proc ?
        str.value.sub(search, &replace) :
        str.value.sub(search, replace)
    end
    
    Sass::Script::String.new(val)
  end
end

module Sass::Script::Functions
  include WakuworksFunctions
end