In Java regular expressions, you can use ^ to match the start of a string and $ to match the end of a string.

^ matches the start of a string, so if you place it at the beginning of a regular expression, the regular expression will only match if the input string starts with the specified pattern.

$ matches the end of a string, so if you place it at the end of a regular expression, the regular expression will only match if the input string ends with the specified pattern.

Here are a few examples:

// Match a string that starts with "Hello"
String input1 = "Hello world";
boolean matches1 = input1.matches("^Hello.*");
System.out.println(matches1); // true

// Match a string that ends with "world"
String input2 = "Hello world";
boolean matches2 = input2.matches(".*world$");
System.out.println(matches2); // true

// Match a string that starts with "Hello" and ends with "world"
String input3 = "Hello world";
boolean matches3 = input3.matches("^Hello.*world$");
System.out.println(matches3); // true

In the first example, the regular expression ^Hello.* matches a string that starts with "Hello" followed by any number of characters.

In the second example, the regular expression .*world$ matches a string that ends with "world" preceded by any number of characters.

In the third example, the regular expression ^Hello.*world$ matches a string that starts with "Hello" followed by any number of characters and ends with "world".