Advanced Regex - Reluctant Qualifiers

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At the other end of the spectrum from greedy qualifiers are reluctant qualifiers, which try to match as little as possible. Reluctant qualifiers are formed by appending? to an existing greedy qualifier. Thus, X+ becomes X+?, X(n,m} becomes X{n,m}?, and so on. Given the pattern \d+? against the candidate string 1234, for example, the resultant match is 1, as Listing 3-7 demonstrates.

Listing 3-7. Reluctant Qualifier Example

import java.util.regex.*;
public class ReluctantExample{
  public static void main(String args[]){
    //define the pattern
    String regex = "(\\d+?)";
    //compile the pattern
    Pattern pattern = Pattern.compile(regex);
    //define the candidate string
    String candidate = "1234";
    //extract a matcher for the candidate string
    Matcher matcher = pattern.matcher(candidate);
    while (matcher.find()){
      //matches once for each digit
      //if this were not an example of a
      //reluctant qualifier, it would match
      //exactly once, and that match would
      //include every digit in the candidate
      //string "1234".
      System.out.println(matcher.group());
    }
    System.out.println("Done");
  }
}

Every time find() is run, it matches as little as possible, because it’s reluctant to match. The Pattern matches exactly four times: once for each digit. If you weren’t using a reluctant qualifier in the Pattern, there would have been a single match for the entire candidate string, namely 1234, because the Pattern would have been greedy and matched as much as possible.

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This article is excerpted from chapter three of Java Regular Expressions Taming the Java.util.regex Engine, written by Mehran Habibi (Apress, 2004; ISBN: 1590591070). Check it out at your favorite bookstore. Buy this book now.