Substring : how to avoid multiple if else statements for replacing userinput dynamically

Question

I have checked few of the existing questions but could not find any matching case.

I have a index.jsp page with ten variables declared and value assigned , an user from UI input text box can provide any value along with our existing variable with '@' prefix

eg 1: userinput = a=@useremail&b=c&d=@city  
    expected output : a=test@email&b=c&d=newyork  
    eg 2: userinput = a=b&c=@firstname  
    expected output : a=b&c=mark

with variable declared as below

String useremail = "test@email.com"
String lastname = "tony"
String firstname = "mark"
String city = "newyork"
String country = "US"

Now I need to replace these variables with actual values , if have tried with below snippet

if (inputText.contains("@")) {
    
    if (inputText.contains("@useremail")) 
        inputText = inputText.replace("@useremail",useremail);
    else if  (inputText.contains("@lastname"))
         inputText = inputText.replace("@lastname", lastname);
    else if  (inputText.contains("@firstname"))
        inputText = inputText.replace("@firstname", firstname);

    //need to check for all variables
}

I find trouble in using switch case as the value is a substring of the userinput, please help me to execute the case with best practices.

Thank you.