Java replace capturing group with different value

Question

In java I want to replace only the capturing group in the matched part with some other value and the value to be replaced depends on the capturing group.

Eg:

String str = "fname:hello hello:world ffff:off";
Map<String, String> mapping = new HashMap<String, String>();
dat.put("fname", "meme");
dat.put("hello", "below");
dat.put("ffff", "ssss");
Pattern pat = Pattern.compile("([a-zA-Z_]+):");

In the above code I want to replace the capturing group part of the pattern "pat" with the corresponding mapping found in the "mapping". i.e. After the replacement the "str" string should be transformed to "meme:hello below:world ssss:off" How can I achieve this? Thanks for the help.

Per map terms would replace whereever the word occcurs. It might work for my example. So in general I want to replace the capturing group of the matched part with some value which depends on the captured group. — Bourne
– Bourne, Commented Apr 16, 2014 at 4:00

Community · Accepted Answer · 2017-05-23 12:20:47Z

5

Please consider bookmarking the Stack Overflow Regular Expressions FAQ for future reference. There's a bunch of Java-specific information in there, particularly in the "Flavor-Specific Information" section.

This works:

import  java.util.Map;
import  java.util.HashMap;
import  java.util.regex.Pattern;
import  java.util.regex.Matcher;

public class Replacement  {
   public static final void main(String[] ignored)  {

      String str = "fname:hello hello:world ffff:off";
      Map<String, String> mapping = new HashMap<String, String>();
      mapping.put("fname", "meme");
      mapping.put("hello", "below");
      mapping.put("ffff", "ssss");
      Pattern pat = Pattern.compile("([a-zA-Z_]+):");

      Matcher m = pat.matcher(str);


      StringBuffer sb = new StringBuffer();
      while(m.find())  {
         String rplcWith = mapping.get(m.group(1));
         m.appendReplacement(sb, rplcWith + ":");
      }
      m.appendTail(sb);

      System.out.println(sb);
   }
}

Output:

[C:\java_code\]java Replacement
meme:hello below:world ssss:off

edited May 23, 2017 at 12:20

CommunityBot

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answered Apr 16, 2014 at 3:49

aliteralmind

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1 Comment

aliteralmind Over a year ago

Sweet! Thank you for the up-vote, and the big green checkmark! :)

merlin2011 · Accepted Answer · 2014-04-16 03:54:34Z

1

If you are just trying to match a word if it appears before a :, it is much easier to just create that expression and iterate over the map.

import java.util.*;

public class Main{ 
    public static void main(String[] args) {
        String str = "fname:hello hello:world ffff:off";
        Map<String, String> mapping = new HashMap<String, String>();
        mapping.put("fname", "meme");
        mapping.put("hello", "below");
        mapping.put("ffff", "ssss");

        for (String key: mapping.keySet()) 
            str = str.replace(key + ":", mapping.get(key)+":");
        System.out.println(str);
    }
}

This is simple but sufficient to get your desired output:

meme:hello below:world ssss:off

answered Apr 16, 2014 at 3:54

merlin2011

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3 Comments

aliteralmind Over a year ago

@PrathikPuthran: This map-iteration is a simpler and more efficient solution than using regex. Is regex required for some reason? It would be nice to have a bit more context.

Bourne Over a year ago

Its simpler but its not the correct solution for Eg if there is a mapping from "me" -> "xyz" then "fname:hello" would be replaced to "fnaxyz:hello" which is not what I want

merlin2011 Over a year ago

@PrathikPuthran, The given solution works for the input you provided, but if you require a space before the word to make sure it is sitting alone, that is still cheaper computationally than a regex.

Eddie Curtis · Accepted Answer · 2014-04-16 03:49:49Z

0

You could go through the entrySet of your map and replace all instances of the keys with their associated values

for (Entry<String, String> entry : mapping.entrySet()) {
    str.replaceAll(entry.getKey(), entry.getValue();
}

However, this will have a different output string to what you're expecting because the word "hello" appears twice

So instead of meme:hello below:world ssss:off you will get meme:below below:world ssss:off

Does that answer your question adequately or is it essential that the output is how you defined it?

answered Apr 16, 2014 at 3:49

Eddie Curtis

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6 Comments

Bourne Over a year ago

Its very essential output is how I desire it to be!

Eddie Curtis Over a year ago

Then you could do str.replaceAll(entry.getKey() + ":", ":" + entry.getValue()); but that is a bit of a hack so I'd recommend go with a different method :)

Bourne Over a year ago

This won't work for my case as it replaces first "hello" even though it does not match regex

Eddie Curtis Over a year ago

It shouldn't do if you copied it right, as the key is hello and you're concatenating it with ":" so that wouldn't match your first "hello"

Bourne Over a year ago

If there is a mapping from "me" to "abc" your solution would replace "fnaabc:hello" instead of the expected "meme:hello"

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