Question

Is it possible to know if a stream/string contains an input that could match a regular expression.

For example

 String input="AA";
 Pattern pat=Pattern.compile("AAAAAB");
 Matcher matcher=pat.matcher(input);
 //<-- something here returning true ?

or

 String input="BB";
 Pattern pat=Pattern.compile("AAAAAB");
 Matcher matcher=pat.matcher(input);
 //<-- something here returning false ?

Thanks

Answer 1

Yes, Java provides a way to do that. First you have to call one of the standard methods to apply the regex, like matches() or find(). If that returns false, you can use the hitEnd() method to find out if some longer string could have matched:

String[] inputs = { "AA", "BB" };
Pattern p = Pattern.compile("AAAAAB");
Matcher m = p.matcher("");
for (String s : inputs)
{
  m.reset(s);
  System.out.printf("%s -- full match: %B; partial match: %B%n",
                    s, m.matches(), m.hitEnd());
}

output:

AA -- full match: FALSE; partial match: TRUE
BB -- full match: FALSE; partial match: FALSE