How can I perform a partial match with java.util.regex.*?

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You should have looked more closely at the Matcher API; the hitEnd() method works exactly as you described:

import java.util.regex.*;

public class Test
{
  public static void main(String[] args) throws Exception
  {
    String[] ss = { "aabb", "aa", "cc", "aac" };
    Pattern p = Pattern.compile("aabb");
    Matcher m = p.matcher("");

    for (String s : ss) {
      m.reset(s);
      if (m.matches()) {
        System.out.printf("%-4s : match%n", s);
      }
      else if (m.hitEnd()) {
        System.out.printf("%-4s : partial match%n", s);
      }
      else {
        System.out.printf("%-4s : no match%n", s);
      }
    }
  }
}

output:

aabb : match
aa   : partial match
cc   : no match
aac  : no match

As far as I know, Java is the only language that exposes this functionality. There’s also the requireEnd() method, which tells you if more input could turn a match into a non-match, but I don’t think it’s relevant in your case.

Both methods were added to support the Scanner class, so it can apply regexes to a stream without requiring the whole stream to be read into memory.

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