How can one make a conditional regex replacement in Matcher.replaceAll()?

I have a file path that I break into groups (using a regex statement). Each group is separated by a _. Many of the groups are optional groups. It looks like this:

Pattern pattern = Pattern.compile(regEx);
Matcher matcher = pattern.matcher(path.toLowerCase());

if (matcher.find())
{
    String type = matcher.replaceAll("$2_$3");
    return type.replaceAll("-","_");
}

So, let's say I have a path: hello_world_20190221.xml.json.
Let us also say this is my regex:

(.*?)_(ce)?_?(.*)_([0-9]{8})\.([A-Za-z]{1,20})?\.([A-Za-z]{1,20})?.

Currently, the matcher.replaceAll("$2_$3") statement will return to me type=_world. I need the replacement string to have condition logic. If $2 is empty, return only $3. Otherwise, return $2_$3. Does the replaceAll() function support condition regex logic? I have not had any success with it.

The purpose of this particular piece of code is to consume a path, a regex, and an expression for the replaceAll() function. It returns a group or some concatenation of groups. I cannot change this code to include if statements and the like. but I can change the code in any other way, I think. So, if using a different function all together will do what I want, please suggest one. This piece of code has many different scenarios running through it, so I cannot plan for just one scenario (such as putting an if-statement to check if group 2 is empty or not). All that conditional logic must come through one of those three strings: path, regex, or the replacementString. I can change the function all together, but cannot make it explicitly (through the source code) check to see if thing condition or that condition is met.

Am I making any sense?

You can modify your regex to combine optional $2 and $3 in same captured group like this:

^([^_]*)_((?:ce_)?[^_]*)_([0-9]{8})\.([A-Za-z]{1,20})?\.([A-Za-z]{1,20})?    

Then you just need to use:

if (matcher.find()) {
    String type = matcher.replaceAll("$2");
    return type.replaceAll("-","_");
}

RegEx Demo

Java code based solution:

> need the replacement string to have condition logic. If $2 is empty, return only $3. Otherwise, return $2_$3

Since you already have a matcher, you can do this:

if (matcher.find()) {
    String type = (matcher.group(2) == null || matcher.group(2).isEmpty() ?
       matcher.replaceAll("$2") :
       matcher.replaceAll("$2_$3"));
    return type.replaceAll("-","_");
}

Use simply (ce_?)?(.*), and replace by $2$3. If $2 is empty, $2$3 will be $3, else, it will be like $2_$3 :

(.*?)_(ce_?)?(.*)_([0-9]{8})\.([A-Za-z]{1,20})?\.([A-Za-z]{1,20})?

hello_ce_world_20190221.xml.json // ce_world
hello_world_20190221.xml.json    // world