Java RegEx with lookahead failing

Question

In Java, I was unable to get a regex to behave the way I wanted, and wrote this little JUnit test to demonstrate the problem:

public void testLookahead() throws Exception {
    Pattern p = Pattern.compile("ABC(?!!)");
    assertTrue(p.matcher("ABC").find());
    assertTrue(p.matcher("ABCx").find());
    assertFalse(p.matcher("ABC!").find());
    assertFalse(p.matcher("ABC!x").find());
    assertFalse(p.matcher("blah/ABC!/blah").find());

    p = Pattern.compile("[A-Z]{3}(?!!)");
    assertTrue(p.matcher("ABC").find());
    assertTrue(p.matcher("ABCx").find());
    assertFalse(p.matcher("ABC!").find());
    assertFalse(p.matcher("ABC!x").find());
    assertFalse(p.matcher("blah/ABC!/blah").find());

    p = Pattern.compile("[A-Z]{3}(?!!)", Pattern.CASE_INSENSITIVE);
    assertTrue(p.matcher("ABC").find());
    assertTrue(p.matcher("ABCx").find());
    assertFalse(p.matcher("ABC!").find());
    assertFalse(p.matcher("ABC!x").find());
    assertFalse(p.matcher("blah/ABC!/blah").find()); //fails, why?

    p = Pattern.compile("[A-Za-z]{3}(?!!)");
    assertTrue(p.matcher("ABC").find());
    assertTrue(p.matcher("ABCx").find());
    assertFalse(p.matcher("ABC!").find());
    assertFalse(p.matcher("ABC!x").find());
    assertFalse(p.matcher("blah/ABC!/blah").find());  //fails, why?
}

Every line passes except for the two marked with the comment. The groupings are identical except for pattern string. Why would adding case-insensitivity break the matcher?

Bart Kiers · Accepted Answer · 2011-02-17 22:56:56Z

1

Your tests fail, because in both cases, the pattern [A-Z]{3}(?!!) (with CASE_INSENSITIVE) and [A-Za-z]{3}(?!!) find at least one match in "blah/ABC!/blah" (they find bla twice).

A simple tests shows this:

Pattern p = Pattern.compile("[A-Z]{3}(?!!)", Pattern.CASE_INSENSITIVE);
Matcher m = p.matcher("blah/ABC!/blah");
while(m.find()) {
    System.out.println(m.group());
}

prints:

bla
bla

edited Feb 17, 2011 at 22:56

answered Feb 17, 2011 at 22:37

Bart Kiers

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1 Comment

romacafe Over a year ago

DUH! All answers are correct, and I'm feeling sheepish for not seeing it myself. This answer gets the win for also telling me how I should have found it on my own...

CanSpice · Accepted Answer · 2011-02-17 22:32:35Z

1

Those two don't throw false values because there are substrings within the full string that match the pattern. Specifically, the string blah matches the regular expression (three letters not followed by an exclamation mark). The case-sensitive ones correctly fail because blah isn't upper-case.

answered Feb 17, 2011 at 22:32

CanSpice

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