code: function(sass) {
      return sass.replace(/\$([\w-]+)\s*([^:;\s]*;)/g, function(match, fieldName, terminus) {
        if (this.vars.hasOwnProperty(fieldName)) {
          this._addFieldUsage(fieldName);
          return this.fieldOpen + fieldName + this.fieldClose + terminus;
        }
        return match;
      }.bind(this));
    }
docs: Replaces parsed variable names into field identifiers.
  Example: "$color-primary" >> "____color-primary____"
  This method may be used as a plain-text fallback when AST parsing fails.
  @param {String} sass string to find and replace variable names within.
  @returns {String} the rendered string with variables formatted as fields.