Replacing Variables with Data in Java

String.format("The user %s is %s 50 years old.", a[0],a[1]);

If you must use <<a1>> and <<a2>> then something like this instead...

String s = "The user <<a1>> is <<a2>> years old.";
String output = s.replace("<<a1>>", a[0]).replace("<<a2>>", a[1]);

If you don't have control over the string, you can replace instances of << a# >> as follows:

String yourString = "The user << a1 >> is << a2 >> years old.";
String transformedString = yourString.replaceAll("<< a\d >>", "%s");

This will replace any instance of << a# >> with %s. That will change your "The user << a1 >> is << a2 >> years old." string to "The user %s is %s years old" which can be used with the string formatter.

Then use the string formatter as follows.

String finalString = String.format(transformedString, a);

There's nothing stopping you from changing the format of your input string, before processing it further. It would give you more options.

That said, this uses a "reluctant" regex to replace all "gaps" as you require. It assumes that there's exactly one character before the index number, but the index can be multiple digits.

 import  java.util.regex.Pattern;
 import  java.util.regex.Matcher;
 /**
    <P>{@code java FillGaps}</P>
  **/
 public class FillGaps  {
    public static final void main(String[] ignored)  {

       String sRegex = "<< (.+?) >>";

       String sToSearch = "The user << a1 >> is << a2 >> years old.";
       String[] values = new String[] {"John", "30"};

       Matcher m = Pattern.compile(sRegex).matcher(sToSearch);
       StringBuffer sb = new StringBuffer();

       while(m.find())  {
          String gapName = m.group(1);
          String idxPlus1AsStr = gapName.substring(1, gapName.length());
          int arrayIdx = Integer.parseInt(idxPlus1AsStr) - 1;
          m.appendReplacement(sb, values[arrayIdx]);
       }
       m.appendTail(sb);

       System.out.println(sb);
    }
 }

Output:

[C:\java_code\]java FillGaps
The user John is 30 years old.