Liferay uses AlloyUI taglibs for front-end development and it provides inbuilt input field validation rules.

Following are inbuild validation rules of alloy liferay :

Here’s a list of form validation rules available for AlloyUI’s FormValidator:

acceptFiles: Define accepted file types (Default:empty)

alpha: Input field should contain only alphabetic characters (Default:none)

alphanum: Input field should contain only alphanumeric characters (Default:false)

date: Input field should contain only a date (Default:false)

digits: Input field should contain only digits (Default:false)

email: Input field should contain only an email address (Default:false)

equalTo: Confirm input field’s values are equal to the specified value. (Default:empty)

iri: Input field should contain only an International Rough Index (Default:false)

max: Integer value is greater than the value. (Default:none)

maxLength: Input field’s contents are greater than the count of characters specified (Default:empty)

min: Determines if the integer value is less than the specified value (Default:none)

minLength: Input field’s contents are less than the count of characters specified (Default:empty)

number: Input field should contain only numeric values (Default:false)

range: Integer value of the field is within the specified range (Default:none)

rangeLength: Input field’s contents is within the specified range (Default:empty)

required: Input field is required for submission (Default:false)

url: Input field’s contents are a URL (Default:false)

Now, Below example help to create custom form validation 

Below example blocks the given email addresses:

<aui:input  label="" name="emailAddress" placeholder="EMAIL">
       <aui:validator name="custom"            
             errorMessage="Oops! We need a personal email address for access to the system.">
            function (val, fieldNode, ruleValue)
                 {
                  var result = true;
                   var blockEmail=["abuse@","admin@","billing@","compliance@","devnull@","dns@","ftp@","hostmaster@","inoc@","info@","ispfeedback@","ispsupport@","list-request@","list@","maildaemon@","noc@","no-reply@","noreply@","null@","phish@","phishing@","postmaster@","privacy@","registrar@","root@","security@","spam@","support@","sysadmin@","tech@","undisclosed-recipients@","unsubscribe@","usenet@","uucp@","webmaster@","www@"];
         var length = blockEmail.length;
        while(length--) {
            if (val.toLowerCase().indexOf(blockEmail[length])!=-1) {
            result= false;    
                       }
         }
                val=$.trim(val);
                $("#<portlet:namespace />emailAddress").val(val);
                return result;
              }
       </aui:validator>
</aui:input>