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

    public class Regex {

        public static void main(String args[]){     

            Pattern p = Pattern.compile(".*?(cat).*?(dog)?.*?(tiger)");
            String input = "The cat is a tiger";
            Matcher m = p.matcher(input);
            StringBuffer str = new StringBuffer();
            if (m.find()) {

            //In the output i want to replace input string with group 3 with group 1 value and group 2 with cow. Though group2 is present or not.
//i.e. group 2 is null
            }
        }
    }

I'd like to know whether it is possible in java to use regex to replace an input string with a particular value for captured groups.

Please help

share|improve this question
    
I'm having difficulty understanding your requirements. Can you give us a couple of examples to illustrate? For example, what do you expect the output to be when the input is "The cat is a tiger"? And what do you expect the output to be when the input is "My dog ate a tiger"? –  David Wallace Nov 6 '13 at 10:51
1  
Please elaborate your requirements, if you know what has to be replaced and with what value, you can use String.replace(target, replacement) –  lazyprogrammer Nov 6 '13 at 10:53
    
i want output like this. "The cat is a cow tiger". (Though the sentence is meaningless. i'm here explaining context. ) Also if input is "The cat is a dog and a tiger. Then output will be " The cat is a cow and a cat" –  Mani Nov 6 '13 at 10:54
    
so did anything help? –  lazyprogrammer Nov 6 '13 at 11:24
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2 Answers

up vote 0 down vote

The String class's replace and replaceAll methods are the best way to do this. They support regex strings as the search argument.

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replace() actually replaces each EXACT match of the given string or char. replaceFirst() and replaceAll() can be used with regex. –  grexter89 Nov 6 '13 at 11:30
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up vote 0 down vote 
Pattern p = Pattern.compile("(cat)(.*?)(dog)?(.*?)(tiger)");
String input = "The cat is a tiger";
Matcher m = p.matcher(input);
StringBuffer str = new StringBuffer();
while(m.find())
{
  m.appendReplacement(str, "$5$2$3$4$1");
}
m.appendTail(str);
System.out.println(str);

By the way if it doesn’t matter whether there’s a dog you can simplify it:

Pattern p = Pattern.compile("(cat)(.*?)(tiger)");
String input = "The cat is a tiger";
Matcher m = p.matcher(input);
StringBuffer str = new StringBuffer();
while(m.find())
{
  m.appendReplacement(str, "$3$2$1");
}
m.appendTail(str);
share|improve this answer
    
Just as a side note: That’s the equivalent to String result="The cat is a tiger".replaceAll("(cat)(.*?)(dog)?(.*?)(tiger)", "$5$2$3$4$1"); so if you want to do it just once that single line will do too. –  Holger Nov 6 '13 at 11:39
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