The appendReplacement and appendTail

[Pradeep bhatt]

What is the difference b/w appendReplacement and replaceAll methods in the java.util.regex.Matcher class? Thanks
[ December 17, 2004: Message edited by: Pradeep Bhat ]

[Alan Moore]

appendReplacement and appendTail are the methods that do the real work when you call replaceAll. Each time a match is found in the input, appendReplacement appends to the output all the text between the previous match and the current one, then it appends the replacement text. appendReplacement is also the method that processes the replacement text, converting $1, $2, etc. to the corresponding captured groups. When there are no more matches, appendTail appends whatever's left of the input (whatever followed the last match).

You use appendReplacement and appendTail directly when you want to process the input in some other--or additional--manner than replaceAll does. For instance, if you want to use the matched text as a key to look up the replacement value in a table, you have to write your own replaceAll equivalent. Here's an example I happened to have handy:

import java.util.*;
import java.util.regex.*;
 
class Test
{
  static Map props = new HashMap();
  static
  {
    props.put("key1", "fox");
    props.put("key2", "dog");
  }
 
  public static void main(String[] args)
  {
    String input = "The quick brown ${key1} jumps over the lazy ${key2}.";
 
    Pattern p = Pattern.compile("\\$\\{([^}]+)\\}");
    Matcher m = p.matcher(input);
    StringBuffer sb = new StringBuffer();
    while (m.find())
    {
      m.appendReplacement(sb, "");
      sb.append(props.get(m.group(1)));
    }
    m.appendTail(sb);
    System.out.println(sb.toString());
  }
}

[Pradeep bhatt]

Thanks Alan .

[Nick Widelec]

I was searching for a good explanation of the method appendReplacement, further than the above posted there is this one which is very very good in my opinion.

http://www.java2s.com/Code/Java/Regular-Expressions/MatcherappendReplacement.htm

[Henry Wong]

Nick Widelec wrote:I was searching for a good explanation of the method appendReplacement, further than the above posted there is this one which is very very good in my opinion.

http://www.java2s.com/Code/Java/Regular-Expressions/MatcherappendReplacement.htm

To be blunt, this example is pretty bad. It is not only *NOT* "very very good", it is not even a valid example as it is being used wrong.

The appendReplacement() method is paired up with the appendTail() method. This example doesn't do that. And if you look at the output, you will see that it does more than just a simple replacement. It also truncates everything after the match (due to the missing appendTail() call). Unfortunately, it is only a period that has been truncated -- which probably went unnoticed.

Anyway, if this is a sign of the quality, I would highly recommend that you use a different site to learn Java.

Henry

[gMale Another]

The code above from Alan Moore has two problems:
1) it doesn't take advantage of appendReplacement properly
2) it doesn't account for null pointers or unmatched properties

Here is a slight tweak to the code that resolves those issues:

	public String replaceVariables(String input, Map<String, String> context) {
		if(context == null || input == null)
			return input;

		Matcher m = Pattern.compile( "\\$\\{([^}]*)\\}" ).matcher( input );
		// Have to use a StringBuffer here because the matcher API doesn't accept a StringBuilder -kg
		StringBuffer sb = new StringBuffer();
		while(m.find()) {
			String value = context.get(m.group(1));
			if(value != null)
				m.appendReplacement(sb, value);
		}
		m.appendTail(sb);
		return sb.toString();
	}

Of course, nowadays, there are plenty of frameworks that handle this kind of thing for us like Spring's PropertyPlaceholderHelper.