核心代码不足50行的表单验证，支持ajax验证和表单验证，支持自定义规则和自定义提醒样式http://www.oschina.net/code/snippet_1015196_32669

validate.js

/*
 * @author duansong
 * @email  duansong@baidu.com
 * @date   2014-1-14
 * @descrition 基于jquery的表单验证组件,支持ajax验证
 */
(function($,window){
		//配置文件
	var settings = {
	   //验证规则
		rules:{
			required:{
				message:"该字段必须填写",
				rule:"\\S+"
			},
			number:{
				message:"只能输入数字",
				rule:"^\\d*$"
			},
			chinese:{
				message:"只能输入中文字符",
				rule:"^[\\u4e00-\\u9fa5]*$"
			},
			email:{
				message:"请输入一个正确的邮箱地址",
				rule:"\\w+([-+\\.]\\w+)*@\\w+([-\\.]\\w+)*\\.\\w+([-\\.]\\w+)*" 
			},
			url:{
				message:"请输入一个正确的url",
				rule:"[a-zA-z]+://[^\\s]*"
			},
			phone:{
				message:"请输入一个正确的电话号码",
				rule:"\\d{3}-\\d{8}|\\d{4}-\\d{7}|\\d{11}"
			},
			zipcode:{
				message:"请输入一个正确的邮政编码",
				rule:"[1-9]\\d{5}(?!\\d)"
			},
			identificate:{
				message:"请输入一个正确的身份证号码",
				rule:"\\d{15}|\\d{18}"
			},
			range:{
				message:"只能输入{0}-{1}位数字",
				rule:"^\\d{{0},{1}}$"
			},
			price:{
				message:"请输入有效的价格",
				rule:"^[0-9]+(\\.[0-9]{1,2})?$"
			}
		},
		/*
		 * @param isPass是否通过验证
		 * @message 未通过验证的提示信息 
		 * */
		placeholder:function(isPass,message){
			if(isPass){
				var error = $(this).next(".valid-error");
				error.length&&$(this).next(".valid-error").remove();
				$(this).removeClass("el-valid-error");
			}else{
				$(this).addClass("el-valid-error");
				var error = $(this).next(".valid-error");
				if(!error.length){
					 $(this).after("<label class='valid-error'></label>");
					 error = $(this).next(".valid-error");
				}
				error.text(message);
			}
			
		}
	};
	//字符串format
	//例如"我是{11}"
	//如果只传入source，则返回11（数组）
	//如果传入source，params，则替换{}中内容
	var format = function(source,params){
		if(typeof params =="undefined"){
			var params = [];
			var b = source.match(/\{+(\S+?)\}+/g);
			b = $.map(b,function(n){
				return n.replace(/\{|\}/g,"");
			});
			return b;
		}else{
			params = $.isArray(params)?params:[params];
			$.each(params, function( i, n ) {
				 source = source.replace( new RegExp("\\{" + i + "\\}", "g"), n);
			});
			return source;	
		}
	}
	//检查元素属性是否通过验证
	var	check = function(el,rules){
		var val = $(el).attr("data-valid").split(";");
		var errorMsg = "",isPass=true;
		for(var i=0,max=val.length;i<max;i++){
			if(val[i]=="")continue;
			v=val[i].split(":");
			var para = [];
			if(v.length>1){
				para = format(v[1]);
			}
		  	var rule= format(rules[v[0]]["rule"],para);
		  	var value = $(el).val();
			if(!new RegExp(rule).test(value)){
				if(v[0]!=="required"&&value==""){
					continue;
				}else{
					errorMsg = format(rules[v[0]]["message"],para);
					isPass = false;
					break;
				}	
			}else{
				continue;
			} 
		}
		return {isPass:isPass,errorMsg:errorMsg};
	}
	//定义扩展方法，用于特殊扩展的处理
	var extend = function(oObj,nObj){
		var result ={};
		$.extend(result,oObj);
		if($.isPlainObject(nObj)){
			$.each(nObj,function(n,v){
				if(n=="rules"){
					$.extend(result.rules,nObj.rules);
				}else{
					result[n] = nObj[n];
				}
			});
		}
		return result;
	}
	var Valid = function(node,opts){
		var setting = extend(settings,opts);
		var $formNode = $(node);
		var valid = true;
		$("[data-valid]",$formNode).each(function(){
			var result = check(this,setting.rules);
			if(result.isPass===false){
				valid = false;
			}
			setting.placeholder.call(this,result.isPass,result.errorMsg);
		});
		 $formNode.delegate("[data-valid]","blur",function(){
		 	var result = check(this,setting.rules);
			setting.placeholder.call(this,result.isPass,result.errorMsg);
		 });
		return valid;
	}
	var Validate = function(node,opts){
		var setting = extend(settings,opts);
		var $formNode = $(node);
		 $formNode.submit(function(){
			 return Valid($formNode,opts);
		 });
	}
	//对外接口
	$.extend($.fn,{
		//一般用于表单验证
		validate:function(opts){
			return this.each(function(){
				Validate(this,opts);
			});
		},
		//一般用于ajax验证，callback回调函数
		valid:function(opts,callback){
			if($.isFunction(opts)&&typeof callback =="undefined"){
				callback = opts;
				opts = {};
			}
			return this.each(function(){
				var isPass = Valid(this,opts);
				$.isFunction(callback)&&callback.call(window,isPass);
			});
		}
	});
	
})(jQuery,window)
//css
.el-valid-error{border:1px solid #B92C28}
.valid-error{color:#B92C28;font-size:12px}