How to check if a character is a non-word boundary

Question

In Java regular expression, it has "\B" as a non-word boundary.

http://java.sun.com/j2se/1.4.2/docs/api/java/util/regex/Pattern.html

If I have a 'char', how can I check it is a non-word boundary?

Thank you.

Answer 1

The boundary has a special meaning. It has actually a zero-length match and can therefore not be matched on a single character. It is used to determine the position between a non-word char and a word-char. Also see http://regular-expressions.info/wordboundaries.html.

I however understood that this question is more whether the given char can possibly denote the start or end of a word boundary. From the javadoc which you linked (here is the latest version):

Predefined character classes

. Any character (may or may not match line terminators)
\d A digit: [0-9]
\D A non-digit: [^0-9]
\s A whitespace character: [ \t\n\x0B\f\r]
\S A non-whitespace character: [^\s]
\w A word character: [a-zA-Z_0-9]
\W A non-word character: [^\w]

So, a word character matches \w. A non-word character matches \W. So:

String string = String.valueOf(yourChar);
boolean nonWordCharacter = string.matches("\\W");

Answer 2

((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'))

or if you want to digits to be also parts of a word:

((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9'))

Answer 3

A boundary is a position between two characters, so a character can never be a boundary.

If you want to match a character that is not surrounded by word boundaries, e. g. the character b in abc, then you can use

\B.\B

Remember to escape the backslashes in a Java string, as in

Pattern regex = Pattern.compile("\\B.\\B");

Answer 4

The question is very peculiar, but it's true that a \w on its own is surrounded by \b. Similarly, a \W on its own is surrounded by \B. So for the purpose of word boundary definitions, ^ and $ are non-word characters.

    System.out.println("a".matches("^\\b\\w\\b$")); // true
    System.out.println("a".matches("^\\b\\w\\B$")); // false
    System.out.println("a".matches("^\\B\\w\\b$")); // false
    System.out.println("a".matches("^\\B\\w\\B$")); // false

    System.out.println("@".matches("^\\b\\W\\b$")); // false
    System.out.println("@".matches("^\\b\\W\\B$")); // false
    System.out.println("@".matches("^\\B\\W\\b$")); // false
    System.out.println("@".matches("^\\B\\W\\B$")); // true

    System.out.println("".matches("$$$$\\B\\B\\B\\B^^^")); // true

The last line may be surprising, but such is the nature of anchors.

See also

• regular-expressions.info/Anchors