var getCharByTagName = function(str, tagName){
  var reg  = /\[([^\]]+)\](.*)\[\\(\1)\]/;
  var result;
  if((result = reg.exec(str)) && result[1] === tagName) {
    return result[2];
  }
  return false;
}   
console.log(getCharByTagName('[cn]中文[\\cn]', 'cn'));

------解决方案--------------------
function getMatchs(str,tag){
var reg =eval('/\\['+tag+'\\](.+?)\\[\\/'+tag+'\\]/i');
return str.match(reg);
}
var m = getMatchs('aaaa[cn]中文[/cn]bbbb[en]英文[/en]ccccc','cn');
alert(m.length +"\r\n" +m[0]+"\r\n"+m[1]);

------解决方案--------------------
JScript code

function getResult(str, tagName)
{
    var regex = "\\["+tagName+"\\]([^\\[]*)\\[/"+ tagName +"\\]";
    var oRegex = new RegExp(regex,'ig');
    var result = [];
    var match;
    while(match = oRegex.exec(str))
    {
        result.push(match[1]);
    }
    return result;
}

function getCharByTagName(str,tagName)
{
    var result = getResult(str,tagName);
    if(result.length > 0) return result.join(',');
    else return '没有匹配的结果';
}


var result = getCharByTagName('[en]中文[/en][en]英文[/en]','en');

alert(result)