When does '.' not match in a Regex?

Question

I encountered the following problem (simplified). I wrote the following

Pattern pattern = Pattern.compile("Fig.*");
String s = readMyString();
Matcher matcher = pattern.matcher(s);

In reading one string the matcher failed to match even though it started with "Fig". I tracked the problem down to a rogue character in the next part of the string. It had codePoint value 1633 from

(int) charAt(i)

but did not match the regex. I think it is due to a non-UTF-8 encoding somewhere in the input process.

The Javadocs say:

Predefined character classes . Any character (may or may not match line terminators)

Presumably this is not a character in the strict sense of the word, but is is still part of the String. How do I detect this problem?

UPDATE: It was due to a (char)10 which was not easy to spot. My diagnosis above is wrong and all answers below are relevant to the question as asked and are useful.

Answer 1

It's easy enough to check this:

import java.util.regex.*;

public class Test {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile(".");
        for (char c = 0; c < 0xffff; c++) {
            String text = String.valueOf(c);
            if (!pattern.matcher(text).matches()) {
                System.out.println((int) c);
            }
        }
    }
}

On my box, the output is:

10
13
133
8232
8233

Of these, 10 and 13 are "\n" and "\r" respectively. 133 (U+0085) is "next line", 8232 (U+2028) is "line separator" and 8233 (U+2029) is "paragraph separator".

Note that:

• This doesn't test any Unicode characters outside the basic multi-lingual plane
• It only uses the default options
• It seems to contradict your experience of character 1633 (U+0661)

Answer 2

The . character in a Java regex matches any character except line terminators, unless you use the flag Pattern.DOTALL when compiling your pattern.

To do so, you would use a Pattern like this:

Pattern p = Pattern.compile("somepattern", Pattern.DOTALL);