Regex - Group Value Replacement

Question

I am not sure if this is possible to do, but I need a way to replace a value of a numbered group specified in the my regex expression with a string declared dynamically at runtime, once a match has been made.

Given a simple case, something like...

(/)?([A-Za-z0-9])?(/)?$

I would want to be able to plugin a replacement for group 2.

I am currently using Java's Matcher class.

Answer 1

Yes, that's doable. Check out my answer to this question to see how. In fact, this question probably should be closed as a duplicate.

You'll need to change the regex a little. I can't tell exactly what you're trying to do, so I can't give any specifics, but at the very least you should move all those question marks inside the groups.

(/)?([A-Za-z0-9])?(/)?$  // NO

(/?)([A-Za-z0-9]?)(/?)$  // YES

But it will still match an empty substring at the end of the target string, because everything is optional except the anchor, $. Is that really what you meant to do?

Answer 2

I am not sure if this is possible to do...

Yes, it's possible. See the example below.

I would want to be able to plugin a replacement for group 2.

This demo "plugs in" the .toUpperCase version of group 2 as a replacement.

import java.util.regex.*;

class Main {
    public static void main(String... args) {
        String input = "hello my name is /aioobe/ and I like /patterns/.";
        Pattern p = Pattern.compile("(/)([A-Za-z0-9]+)(/)");
        Matcher m = p.matcher(input);
        StringBuffer sb = new StringBuffer();
        while (m.find()) {
            String rep = m.group(1) + m.group(2).toUpperCase() + m.group(3);
            m.appendReplacement(sb, rep);
        }
        m.appendTail(sb);
        System.out.println(sb);
    }
}

Prints:

hello my name is /AIOOBE/ and I like /PATTERNS/.

Answer 3

Return the value of your regex search and save it to a variable, then do a replace on your main string using your regex search results as a find target and your dynamically declared string as a replacement.

Really simplified conceptual:

String testString = "Hello there";
//String substring = *Do your regex work here*
if(substring.length() > 0) {
   testString.replace(substring, dynamicallyGeneratedString); 
}