We use character classes to search a group of things. The following represents the Character Classes.

Character Classes supported by Java:

[abc] - we are searching for either 'a' or 'b' or 'c'.
[^ abc] - Except 'a','b' and 'c'.
[a-z] - any lower case alphabet symbol
[A-Z] - any upper case alphabet symbol
[a-z A-Z] - any alphabet symbol
[0-9] - Any digit from 0-9
[a-z A-Z 0-9]- Any alphanumeric character
[^a-z A-Z 0-9] - Except alphanumeric characters(Special Characters)

Let's see a demo program by using character classes:

Class RegexDemo3 {
	public static void main(String[] args) {
		Pattern p = Pattern.compile("x");
		Matcher m = p.matcher("a7b@Z#9");
		while (m.find()) {
			System.out.println(m.start() + "....." + m.end());
		}
	}
}

Output

Pre-defined character classes:

\s: space character
\S: Except space character, any character
\d: Any digit from 0 to 9
\D: Except digits any character
\w:Any word character[a-z A-Z 0-9]
\W: Except word character i.e. special character
. : Any symbol including special character also

Let's see a demo program by using pre-defined character classes:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
class RegExDemo2 {
	public static void main(String[] args) {
		Pattern p = Pattern.compile("\\s");
		Matcher m = p.matcher("a7bk@9");
		while (m.find()) {
			System.out.println(m.start() + "..." + m.group());
		}
	}
}

Output