In Java, regular expressions are patterns used to match character combinations in strings. They are used to perform search and replace operations, to check if a string contains a specified pattern, or to split a string into an array of substrings.

The java.util.regex package provides classes and methods for working with regular expressions in Java.

Here is the syntax for creating a regular expression pattern in Java:

Pattern pattern = Pattern.compile("pattern");

Here is an example of how to use regular expressions in Java:

// Create a pattern to match any sequence of digits
Pattern pattern = Pattern.compile("\\d+");

// Create a matcher for the pattern
String input = "There are 100 apples.";
Matcher matcher = pattern.matcher(input);

// Find all occurrences of the pattern in the input string
while (matcher.find()) {
  String match = matcher.group();
  System.out.println(match);
}

The output of this example is:

100

Regular expressions use a special syntax to represent patterns. Some common special characters are:

.: Matches any single character.
*: Matches zero or more of the preceding character or character class.
+: Matches one or more of the preceding character or character class.
?: Matches zero or one of the preceding character or character class.
[]: Matches any single character within the brackets.
[^]: Matches any single character not within the brackets.
\d: Matches any digit (equivalent to [0-9]).
\D: Matches any non-digit (equivalent to [^0-9]).
\s: Matches any whitespace character (equivalent to [ \t\n\r\f\v]).
\S: Matches any non-whitespace character (equivalent to [^ \t\n\r\f\v]).
\w: Matches any word character (equivalent to [a-zA-Z0-9_]).
\W: Matches any non-word character (equivalent to [^a-zA-Z0-9_]).