Here i have written JS code to validate form, for example:
checking empty fields, checking email, password user-name etc.

function valid()
		{
			var name = document.getElementById('name').value;
			var passwd = document.getElementById('password').value;
			var email = document.getElementById('email').value;
			var mobile = document.getElementById('mobile').value;
			var chk_nam=chk_name(name);
			var chk_emai=chk_email(email);
			var chk_passw=chk_passwd(passwd);
			var chk_mobil=chk_mobile(mobile);
			// alert(chk_empty(name,passwd,email,mobile));
			if(!chk_empty())
			{
				alert("Feilds cannot be left blank");
				return false;
			}
			else if (!chk_name(name.trim()))
			{
				alert("Error: Only alphabets and space are allowed ");
				document.getElementById('name').focus();
				document.getElementById('name').style.borderColor = "red";
				document.getElementById('name').style.boxShadow = "0px 0px 7px 2px red";
				return false;
			}
			else if(!chk_email(email.trim()))
			{
				alert("Error: Invalid Email");
				document.getElementById('email').focus();
				document.getElementById('email').style.borderColor = "red";
				document.getElementById('email').style.boxShadow = "0px 0px 7px 2px red";
				return false;
			}
			else if(!chk_passwd(passwd.trim()))
			{
				alert("Error: Password should atleast contain a alphabets,number and special characters and minimun length sholud be 6 characters"+passwd);
				document.getElementById('password').focus();
				document.getElementById('password').style.borderColor = "red";
				document.getElementById('password').style.boxShadow = "0px 0px 7px 2px red";
				return false;
			}
			else if(!chk_mobile(mobile.trim())) 
			{
				alert("Error: Invalid Mobile number");
				document.getElementById('mobile').focus();
				document.getElementById('mobile').style.borderColor = "red";
				document.getElementById('mobile').style.boxShadow = "0px 0px 7px 2px red";
				return false;
			}
			else
				return true;
		}
		function chk_empty()
		{
			var i=0;
			var flag=false;;
			for(i=0;i<4;i++)
			{
				if(document.getElementsByTagName('input')[i].value=='')
				{
					document.getElementsByTagName('input')[i].focus();
					document.getElementsByTagName('input')[i].style.borderColor = "red";
					document.getElementsByTagName('input')[i].style.boxShadow = "0px 0px 7px 2px red";
					flag=false;
				}
				else{
					document.getElementsByTagName('input')[i].style.borderColor = "#222";
					document.getElementsByTagName('input')[i].style.boxShadow = "none";
					flag=true;
				}
			}
			return flag;
		}
		function chk_name(name)
		{
			var pattern = /^[A-z\s]*$/;
			if(pattern.test(name))
				return true;
			else
				return false;
		}
		function chk_email(email)
		{
			var pattern = /^[a-zA-Z]+([\._] [a-z0-9]+)*@[a-z]+([\.]?[a-z]+)*(\.[a-z]{2,4})+$/;
			if(pattern.test(email))
				return true;
			else
				return false;
		}
		function chk_passwd(passwd)
		{
			/*(/^
        	(?=.*\d)                //should contain at least one digit
        	(?=.*[a-z])             //should contain at least one lower case
        	(?=.*[A-Z])             //should contain at least one upper case
        	(?=.*[!@#$%^&*])		//should contain at least one special characters
        	[a-zA-Z0-9]{8,}         //should contain at least 8 character from the mentioned characters

        	$/)*/
			
			var pattern = /^(?=.*\d)(?=.*[a-zA-Z])(?=.*[!@#$%^&*_])[a-zA-Z0-9_!@#$%^&*]{6,}$/;
			if (pattern.test(passwd))
				return true;
			else
				return false;
		}
		function chk_mobile(mobile)
		{
			var pattern = /^[7-9]+[0-9]*$/;
			if(pattern.test(mobile))
				return true;
			else
				return false;
		}

As you can see i have made a different function checking different type of validation.

Here i have regular expression to perform pattern-matching for various inputs that user may enter.