Java Cookbook

Using Regular Expressions in Java

Problem

You’re ready to utilize regular expression processing to beef up your Java code.

Solution

Use the Apache Jakarta Regular Expressions Package, org.apache.regexp.

Discussion

As mentioned, the Apache project develops and maintains a regular expressions API. To ensure that you get the latest version, I don’t include it in the source archive for this book; you should download it from http://jakarta.apache.org/regexp/. The good news is that it’s actually easy to use. If all you need is to find out whether a given string matches an RE, just construct the RE and call its boolean match( ) method:

RE r = new RE(pattern); // Construct an RE object
boolean found = r.match(input); // Use it to match an input.
if (found) {
    // it matched... do something with it...
}

A complete program constructing an RE and using it to match( ) is shown here:

import org.apache.regexp.*;

/**
 * Simple example of using RE class.
 */
public class RESimple {
    public static void main(String[] argv) throws RESyntaxException {
        String pattern = "^Q[^u]\\d+\\.";
        String input = "QA777. is the next flight. It is on time.";

        RE r = new RE(pattern); // Construct an RE object

        boolean found = r.match(input); // Use it to match an input.

        System.out.println(pattern +
            (found ? " matches " : " doesn't match ") + input);
    }
}

Remember This!

Remember that because an RE will be compiling strings that are also compiled by javac, you will probably need two levels of escaping for any special characters, ...