GWT Regex and empty string - regex

Question

Could someone explain why this snip :
// import com.google.gwt.regexp.shared.MatchResult;
// import com.google.gwt.regexp.shared.RegExp;
RegExp regExp = RegExp.compile("^$");
MatchResult matcher;
while ((matcher = regExp.exec("")) != null)
{
System.out.println("match " + matcher);
}
give an incredible count of matches? I tested with different modifier allowed by GWT implementation of compile(), g, i and m. It works only with m (multiline).
I just want to check for empty string.
[EDIT] the new method
private ArrayList<MatchResult> getMatches(String input, String pattern)
{
ArrayList<MatchResult> matches = new ArrayList<MatchResult>();
if(null == regExp)
{
regExp = RegExp.compile(pattern, "g");
}
if(input.isEmpty())
{
// empty string : just check if pattern validate and
// don't try to extract matches : it will resutl in infinite
// loop.
if(regExp.test(input))
{
matches.add(new MatchResult(0, "", new ArrayList<String>(0)));
}
}
else
{
for(MatchResult matcher = regExp.exec(input); matcher != null; matcher = regExp
.exec(input))
{
matches.add(matcher);
}
}
return matches;
}

Answer 1

Your regExp.exec("") with RegExp.compile("^$") will never return null, as the empty string "" is a match for regex ^$, which reads "nothing between beginning and the end of line/string".
So your while is infinity loop.
Also, you print is
System.out.println("match " + matcher);
...but you probably wanted to use
System.out.println("match " + matcher.getGroup(0));
Also see GWT checking if textbox is empty.