code: function caseInsensitivePart(value) {
      let result = [];
      for(let i=0; i<value.length; i++) {
          const c = value[i];
          const l = c.toLowerCase();
          const u = c.toUpperCase();
  
          if(l !== u) {
              result.push('[' + l + u + ']');
          } else {
              result.push(l);
          }
      }
      return result.join('');
  }
docs: Create a case insensitive part of a regex by taking each letter in a
  string and turning it into `[Aa]`.