Regex NOT operator doesn't work

Question

I'm trying to filter files in a folder. I need the files that don't end with ".xml-test". The following regex works as expected (ok1,ok2,ok3 = false, ok4 = true)

String regex = ".+\\.xml\\-test$";
boolean ok1 = Pattern.matches(regex, "database123.xml");
boolean ok2 = Pattern.matches(regex, "database123.sql");
boolean ok3 = Pattern.matches(regex, "log_file012.txt");
boolean ok4 = Pattern.matches(regex, "database.xml-test");

Now I just need to negate it, but it doesn't work for some reason:

String regex = "^(.+\\.xml\\-test)$";

I still get ok1,ok2,ok3 = false, ok4 = true

Any ideas? (As people pointed, this could be done easily without regex. But for arguments sake assume I have to use a single regex pattern and nothing else (ie !Pattern.matches(..); is also not allowed))

Why did you think that would negate it? (I think you're mistaking [^stuff] for ^stuff$ — Jon Skeet
– Jon Skeet, Commented Nov 6, 2012 at 15:30

Duncan Jones · Accepted Answer · 2012-11-06 15:39:02Z

5

I think you are looking for:

if (! someString.endsWith(".xml-test")) {
  ...
}

No regular expression required. Throw this into a FilenameFilter as follows:

public accept(File dir, String name) {
  return ! name.endsWith(".xml-test");
}

edited Nov 6, 2012 at 15:39

answered Nov 6, 2012 at 15:33

Duncan Jones

69.7k32 gold badges204 silver badges262 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Caner Over a year ago

Thanks, but still I was wondering how can I do this with regex. Do you know what my regex should be?

Duncan Jones Over a year ago

On the basis that you would negate the match, you can search for !Pattern.matches(".+\.xml-test", someString);

Caner Over a year ago

I cannot do that, it's a 3rd party library function, gets a regex pattern as argument and returns a list.(it's a bit more complex situation than i posted here)

Fred Foo Over a year ago

@Caner: regular expression libraries usually don't support negation as (1) it's extremely expensive, taking exponential time to compile the regex and (2) it interferes with things like capturing and non-regular extensions such as backreferences.

Sergey Kalinichenko · Accepted Answer · 2012-11-06 15:32:48Z

4

The meaning of ^ changes depending on its position in the regexp. When the symbol is inside a character class [] as the first character, it means negation of the character class; when it is outside a character class, it means the beginning of line.

The easiest way to negate a result of a match is to use a positive pattern in regex, and then to add a ! on the Java side to do the negation, like this:

boolean isGoodFile = !Pattern.matches(regex, "database123.xml");

answered Nov 6, 2012 at 15:32

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

3 Comments

Caner Over a year ago

Thanks, but still I was wondering how can I do this with regex. Do you know what my regex should be?

Sergey Kalinichenko Over a year ago

@Caner The closest I was able to get to what you are looking for is this expression: "[^.]+\\.(?!xml-test).*". Here is my link to ideone. However, this will give you some "false negatives" when a string would be disqualified because it has ".xml-test" somewhere in the middle. It passes your tests, though.

Caner Over a year ago

ok, I managed to get your version working(changed to this "[^.]+\\.(?!xml-test$).*"), ideone.com/YuhPr0

ridgerunner · Accepted Answer · 2012-11-06 16:11:58Z

2

The following Java regex asserts that a string does NOT end with: .xml-test:

String regex = "^(?:(?!\\.xml-test$).)*$";

This regex walks the string one character at a time and asserts that at each and every position the remainder of the string is not .xml-test.

Simple!

edited Nov 6, 2012 at 16:11

answered Nov 6, 2012 at 16:02

ridgerunner

34.6k6 gold badges60 silver badges70 bronze badges

Comments

hoaz · Accepted Answer · 2012-11-06 15:35:06Z

1

^ - is not a negation in regexp, this is a symbol indicating beginning of line you probably need (?!X) X, via zero-width negative lookahead

But I suggest you to use File#listFiles method with FilenameFilter implementation: name.endsWith(".xml-test")

answered Nov 6, 2012 at 15:35

hoaz

10.2k4 gold badges48 silver badges55 bronze badges

Comments

MBO · Accepted Answer · 2012-11-06 16:08:45Z

1

If you really need to test it with regex, then you should use negative lookbehinds from Pattern class:

String reges = "^.*(?<!\\.xml-test)$"

How it works:

first you match whole string: from start (^) all characters (.*),
you check if what have already matched doesn't have ".xml-test" at end (lookbehind at position you already matched),
you test if it's end of string.

edited Nov 6, 2012 at 16:08

answered Nov 6, 2012 at 15:52

MBO

31.2k5 gold badges52 silver badges52 bronze badges

1 Comment

Caner Over a year ago

This one returns true for all :(

Collectives™ on Stack Overflow

Regex NOT operator doesn't work

5 Answers 5

4 Comments

3 Comments

Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

4 Comments

3 Comments

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related