Confusion in RegExp Reluctant quantifier? Java

Question

Why do I get the output ab for the following regular-expression code with a Relucutant quantifier?

    Pattern p = Pattern.compile("abc*?");
    Matcher m = p.matcher("abcfoo");
    while(m.find())
      System.out.println(m.group()); // ab

Similarly, why do I get empty indices for the following code?

   Pattern p = Pattern.compile(".*?");
   Matcher m = p.matcher("abcfoo");
   while(m.find())
     System.out.println(m.group());

Answer 1

In addition to Konrad Rudolph's answer:

abc*?

matches "ab" in any case and "c" only if it must. Since nothing follows the *?, the regex engine stops immediately. If you had:

abc*?f

then it would match "abcf" be cause the "c" must match in order to allow the "f" to match, too. The other expression:

.*?

matches nothing because this pattern is 100% optional.

.*?f

would match "abcf" again.

Answer 2

*? matches zero or more matches, but as few as possible (and by the way, that’s usually called “non-greedy”, not “reluctant”). So if zero matches is possible, that’s the optimal match.

What exactly do you want to achieve? Maybe non-greedy matching isn’t what you need.