js 正則表達式，動態拼接匹配規則

/**
* 驗證月份表單輸入是否合法
* pattern : 月份格式字符串
* id : 表單id
*/
function validateMonth(pattern, id) {
var text = document.getElementById(id);
var monthStr = text.value;
var splitChar = "";
if(pattern.length > 6) splitChar = pattern.substring(4, pattern.length - 2);
eval("var re = /\\d{4}" + splitChar + "\\d{2}$/;");
//var re = /\d{4}-\d{2}$/;
if(monthStr.match(re) == null) {
alert("請參考格式[" + pattern + "]輸入！\n" + "e.g \"2010" + splitChar + "11\" or \"2010" + splitChar + "03\"");
text.value = "";
text.focus();
return false;
}
return true;
}

需要動態拼出腳本字符串傳給eval()方法執行時，需要對字符'\'轉義