javascript去除前导和后导空格的正则

function trim(input_)
{
    var reg=/(^\s+|\s+$)/gi;
    return  input_.replace(reg, function(march){ 
         return march.replace(/\s+/,""); 
    }); 
}
附replace的实现：
  var GLOBAL = /(g|gi)$/;  
  var _String_replace = String.prototype.replace;   
  String.prototype.replace = function(expression, replacement) {  
    if (typeof replacement == "function") { // Safari doesn't like functions  
      if (expression && expression.constructor == RegExp) {  
       var regexp = expression;  
       var global = regexp.global;  
        //regexp.global 也不可靠？直接判断正则有没有g吧  
        if (global == null) global = GLOBAL.test(regexp);  
        // we have to convert global RexpExps for exec() to work consistently  
       //很精巧，即使是g 全局的也要剥离掉 ,使得全局不全局可以很统一和谐的对待  
        if (global) regexp = new RegExp(regexp.source); // non-global  
      } else {  
        regexp = new RegExp(rescape(expression));  
      }  
      var match, string = this, result = "";  
      while (string && (match = regexp.exec(string))) {  
        //匹配项替换后结果以及匹配前缀放入结果  
        result += string.slice(0, match.index) + replacement.apply(this, match);  
          
        //这样很好，不用像chrome那样还要记录 previous 的状态，直接把要正则匹配的字符           串截断。  
        string = string.slice(match.index + match[0].length);  
          
        //全局不全局只有这一个差别了  
        if (!global) break;  
      }  
        
      //最后一次匹配结束位置到字符串结尾  
      return result + string;  
   }  
    return _String_replace.apply(this, arguments);  
  };