Comprehensive example of php special character filtering

//Filter special characters of string
function checkform() {
 var username = document.theform.user_name.value;
 var realname = document.theform.real_name.value;
 var passwd = document.theform.passwd.value;
 var passwd2 = document.theform.passwd2.value;
 var email = document.theform.email.value;
 var question = document.theform.question.value;
 var answer = document.theform.answer.value;
 var address = document.theform.address.value;
 var phone = document.theform.phone.value;
 var answer = document.theform.answer.value;
 var checkdata = /< ;|>|'|;|&|#|"|'/;
 var checkusername = /[^0-9]/;
 var checkmail = /^([a-za-z0-9_-])+ @([a-za-z0-9_-])+(.[a-za-z0-9_-])+/;
 var checkphone = /[^0-9-]/;
 if ( username == "" ) {
 alert("Username cannot be empty!");
 return false;
 }
 if ( passwd == "" ) {
 alert("Password cannot be empty!");
 return false;
 }
 if ( realname == "" ) {
 alert("Real name cannot be empty!");
 return false;
 }
 if ( question == "" ) {
 alert("Password question cannot be empty!" ");
 return false;
 }
 if ( answer == "" ) {
 alert("The answer to the password question cannot be empty!");
 return false;
 }
 if ( email == "" ) {
 alert("e-mail cannot be empty!");
 return false;
 }
 if ( checkusername.test(username) ) {
 }else{
 alert("Username cannot be all numbers!");
 return false;
 }

 if ( checkdata.test(username) ) {
 alert("The username contains illegal characters, please do not use special characters!");
 return false;
 }
 if ( username.length > 28 || username.length < 3 ) {
 alert("The username length does not meet the requirements [3-28 characters] username.length");
 return false;
 }
 if ( passwd != passwd2 ) {
 alert ("The passwords entered twice are inconsistent!");
 return false;
 }
 if ( passwd.length > 28 || passwd.length < 5 ) {
 alert("The password length does not meet the requirements [5-28 characters]");
 return false;
 }
 if ( checkdata.test(realname) ) {
 alert("The real name contains illegal characters, please do not use special characters!");
 return false;
 }
 if ( realname.length > 28 || realname.length < 3 ) {
 alert("The length of the real name does not meet the requirements [3-28 characters]");
 return false;
 }
 if ( question.length > ; 98 || question.length < 3 ) {
 alert("The password question length does not meet the requirements [6-98 characters]");
 return false;
 }
 if ( answer.length > 98 || answer.length < 3 ) {
 alert("The answer length does not meet the requirements [3-98 characters]");
 return false;
 } //Script Academy http://bbs.it-home.org
 if ( ! checkmail.test(email) ) {
 alert("e-mail contains illegal characters!");
 return false;
 }
 if ( email.length > 48 || email.length < 5 ) {
 alert("The e-mail length does not meet the requirements [5-48 characters]");
 return false;
 }
 if ( checkdata.test(address) ) {
 alert("The contact address contains illegal characters! ");
 return false;
 }
 if ( address != "" && address.length > 48 ) {
 alert("The length of the contact address does not meet the requirements [within 48 characters]");
 return false;
 }
 if ( checkphone.test(phone) ) {
 alert("Contact number contains illegal characters!");
 return false;
 }
 if ( address != "" && ( phone.length > 18 || phone .length < 7 ) ) {
 alert("The length of the contact number does not meet the requirements [7-18 characters]");
 return false;
 }
}

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