To validate and limit the number of words in an input using regular expressions in Java, you can use the following regular expression pattern:

String inputPattern = "^\\s*(\\S+\\s+){0,4}\\S+\\s*$";

This regular expression pattern matches inputs with up to five words. The pattern first matches any number of whitespace characters (including none), then matches zero to four groups of one or more non-whitespace characters followed by one or more whitespace characters. Finally, it matches one group of one or more non-whitespace characters followed by zero or more whitespace characters, and the end of the string.

To use this regular expression to validate an input and limit the number of words, you can use the following code:

String input = "This is a sentence with more than five words.";
if (input.matches(inputPattern)) {
    System.out.println("Valid input: " + input);
} else {
    System.out.println("Invalid input: " + input);
}

In the example above, the "input" string is checked if it matches the "inputPattern" regular expression. If it does, it is considered a valid input with up to five words.

Note that while this regular expression pattern is a good starting point for limiting the number of words in an input, it may not work for all cases. For example, it does not account for the possibility of non-word characters (such as punctuation marks) being included in a word count. It's always a good idea to test your regular expression pattern against various inputs and to use additional validation methods (such as splitting the input into words and counting them) to ensure the input is valid.