Java regex and replacement

Question

Hi I am trying to understand Java regex replacement. I have lots of regex and replacement to apply on text in a file. I want to read regex and apply replacement on text. Like, I want to replace text to variable in following example.

import java.util.regex.*;
public class regex1{
public static void main(String args[]){
    String s1 = "cat catches dog text";
    Pattern p1 = Pattern.compile("\\s*cat\\s+catches\\s*dog\\s+(\\S+)");
    Matcher m1 = p1.matcher(s1);
    if (m1.find()){
        System.out.println(m1.group(1));
        s1 = m1.replaceFirst("variable $1");
        System.out.println(s1);
    }
    else{
        System.out.println("Else");
    }
}    
}

But I get output as

text
variable text

Can any one explain how does group and replacement works in java? How to get correct output?

Answer 1

Use this code:

String s1 = "cat catches dog text";
Pattern p1 = Pattern.compile("\\s*cat\\s+catches\\s*dog\\s+(\\S+)");
Matcher m1 = p1.matcher(s1);
if (m1.find()){
    s1 = m1.replaceFirst(s1.substring(0, m1.start(1)) + "variable");
}
else{
    System.out.println("Else");
}
System.out.println(s1);
// cat catches dog variable

Answer 2

Try this

import java.util.regex.*;
public class regex1{
public static void main(String args[]){
    String s1 = "cat catches dog text";
    Pattern p1 = Pattern.compile("\\s*cat\\s+catches\\s*dog\\s+(\\S+)");
    Matcher m1 = p1.matcher(s1);
    if (m1.find()){
        System.out.println(m1.group(1));
        s1 = s1.replaceFirst(m1.group(1),"variable");
        System.out.println(s1);
    }
    else{
        System.out.println("Else");
    }
}
}