java - newline detection in strings

Question

I know that similar questions have been asked several times, but the answers I saw are either workarounds or simply incorrect.

The basic problem is fairly simple: A method gives me a string read from a file and I need to check if this string contains newline.

Now to the tricky part: Wikipedia currently lists eight types of characters or character combinations which may, dependent on the system, denote a newline. So checking for the common \n and \r, an answer, that I often read, is not the way to go. Walking through the string and compare its characters with System.getProperty("line.separator") might also fail, since a possible newline representation is `\r\n', that will trigger this comparison twice, though its only one newline.

However, this has to be possible. What option am I missing?

Answer 1

You can use the regex pattern ^(.*)$ together with the modifier Pattern.MULTILINE. A method that checks if a string contains any new line character would look like this:

static boolean containsNewLine(String str) {
    Pattern regex = Pattern.compile("^(.*)$", Pattern.MULTILINE);
    return regex.split(str).length > 0;
}

It splits the string in n parts, depending on the number of newline characters. If the string contains any newline character, the length will be greater than 0.

Normally ^ and $ will match only the beginning and the end of the string, but you can change this behavior by passing Pattern.MULTILINE. From the docs:

In multiline mode the expressions ^ and $ match just after or just before, respectively, a line terminator or the end of the input sequence. By default these expressions only match at the beginning and the end of the entire input sequence.

Answer 2

You can try with Regex pattern \r?\n where \r is optional.

sample code:

    String str = "abc\r\nlmn\nxyz";
    Pattern pattern = Pattern.compile("\r?\n");
    Matcher matcher = pattern.matcher(str);
    int count=0;
    while(matcher.find()){
        count++;
    }
    System.out.println(count);    // prints 2