@function __string-replace($string, $target, $replacement: "") {
  $string: __to-string($string);
  $target: __to-string($target);
  $found-index: str-index($string, $target);
  $result: "";

  @if not $found-index {
    @return $string;
  }

  @if __function-exists($replacement) {
    $replacement: __call($replacement, null, $target);
  }

  @return $result + str-slice($string, 1, $found-index - 1) + $replacement +
    __string-replace(
      str-slice($string, str-length($target) + $found-index),
      $target,
      $replacement
    );
}

/// Replaces all instances of a `$target` substring in a `$string` with
/// a `$replacement` string.
///
/// @access public
/// @group String
/// @param {String} $string - The source string.
/// @param {String} $target - The substring to replace.
/// @param {String} $replacement [''] - The replacement string.
/// @returns {String} Returns new string with replacements.
/// @example scss
/// $foo: _replace('abcde', 'bcd', '---');
/// // => 'a---e'
@function _replace($args...) {
  @return call(get-function("__string-replace"), $args...);
}