Java Regex, strange error

Question

I will get a false if running the following code, but if I remove Pattern.CASE_INSENSITIVE from Pattern.compile, the result is true

public static void main(String[] args) {
    Pattern p = Pattern.compile(".*(?<!S)\\.a\\s*\\(\\s*\\)\\s*$", Pattern.CASE_INSENSITIVE);
    String s = "attributes.a()";
    Matcher m = p.matcher(s);
    System.out.println(m.matches());
}

Any idea what's going on?

This question needs a better title and description to be useful to people with the same problem who are looking for the answer. I can't think of a good title though... Could anyone help out? — Anko
– Anko, Commented Aug 22, 2013 at 10:45

Rohit Jain · Accepted Answer · 2013-08-24 22:19:55Z

8

Why is it strange? It is working as intended, perhaps due to the flag you passed as 2^nd argument.

Let's take a look at your pattern: -

".*(?<!S)\\.a\\s*\\(\\s*\\)\\s*$"

Your pattern will match the string which does not have an S before .a. This is clear from the below part of your pattern.

(?<!S)\\.a  // Match if `.a` is not preceded by `S`.

Now, when you use Pattern.CASE_INSENSITIVE, this condition will check for both s and S, before .a and if any of them is present, it will not match it. So your pattern literally becomes: -

(?<![sS])\\.a  // Match if `.a` is not preceded by `S` or `s`.

Now, in your string: -

"attributes.a()"

You have a small s before .a. So, the pattern in your code will return true for this string, with case insensitivity flag enabled.

Just FYI, you can also use (?i) flag instead of passing a second parameter for Pattern.CASE_INSENSITIVE to see the same effect. So the below pattern is same as yours: -

Pattern p = Pattern.compile("(?i).*(?<!S)\\.a\\s*\\(\\s*\\)\\s*$");

The advantage of using (?i) is that, you can use it to make only a part of your pattern CASE_INSENSITIVE. For e.g., if you want that you only check for (?<!S), but the following string can be .a or .A, then you can use (?i) just before .a: -

Pattern p = Pattern.compile(".*(?<!S)(?i)\\.a\\s*\\(\\s*\\)\\s*$");
                                      ^^^

Now, the entire pattern after the (?i) flag will be matched in CASE_INSENSITIVE manner.

Also, note that, you don't really need a look-behind or case-insensitive matching here as pointed out in a comment. You can do it simply by using [^S] and [aA] if your letter is only supposed to be a. Because look-behinds and case-insensitivity brings with them some performance difference. So, you can simply change your pattern to: -

Pattern p = Pattern.compile(".*[^S][.][aA][ ]*[(][ ]*[)][ ]*$");

I also replaced backslashes with character class. I prefer using it, instead of double-escaping the meta-characters. But it's just a matter of taste. You can use whatever you like.

edited Aug 24, 2013 at 22:19

answered Jan 30, 2013 at 6:16

Rohit Jain

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7 Comments

Gelin Luo Over a year ago

Thanks a lot. I actually want to use it to match all xxx.a(), but not S.a(), any idea how to implement it? meaning I want to match to attributes.a() be true, but S.a() should be false

Rohit Jain Over a year ago

Then just don't use case_insensitive. Rest is all fine.

Gelin Luo Over a year ago

I want it to make Attributes().A() to true also

Rohit Jain Over a year ago

@green.. Here comes the need of (?i). The beauty of this flag is that, you can add it at the position, from where you want to apply case_insensitivity. So, you can add it just before .a

Rohit Jain Over a year ago

@Downvoter.. Care to explain your downvote, if you add one. OR post a better solution. And if you can't, then don't downvote.

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