How do I validate a username in a form that should not allow you to use the first name and last name as part of the user name

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Javascript

HTML

i getting the user name to be entered but the first name and last name still submiiting which it not suppose to

What I have tried:

if(username == "" )
{
alert("please enter your username first.");
form_val.username.focus();
return false;

}
else if(username=='fname')
{
alert("username must not contain first name.");
form_val.username.focus();
return false;
}
else if(username=='lname'){
alert("user must not contain last name");
form_val.username.focus();
return false
}