A Java MULTILINE pattern (regex) example

By Alvin Alexander. Last updated: September 3, 2016

Java regex pattern FAQ: Can you share an example of a multiline Java pattern (regular expression)?

Background

I created the following Java code when working on an anti-spam program code-named "Musubi" (which comes from the semi-food product known as Spam Musubi). Through experience — receiving over 1,000 spam messages per day — I've learned that some spammers like to include web addresses like http://10.1.1.1/ in the spam messages they kindly send me. Actually, each numeric value can be any number up to 255, so I need to use Java regular expressions (Java regex patterns) to find text like this within my email messages.

For this to work, what I do in my Java program is extract my email message and any attachments, and make one big Java String of them. Then, I parse that big String using Java regular expressions (regex patterns), as shown below. One important thing that I have to do is use the MULTILINE version of the Java Pattern class, because my big Java String has many newline characters (\n) embedded in it.

(Note that I intentionally embed newline characters in the spamString that I create in the program. Because of these newline characters, the program will not work unless you create the Pattern with the MULTILINE attribute.)

My regex pattern

One other thing to know before working with this program: I require that a parameter be passed in on the command line. That parameter is the regular expression that you want to use to try to match the spamString that is in the program. So, for my tests, I used this simple regular expression:

.*\d+\.\d+\.\d+\.\d+.*

This pattern looks for

any text (including nothing),
followed by one or more decimal characters,
followed by a period,
followed by one or more decimal characters,
followed by a period,
followed by one or more decimal characters,
followed by a period,
followed by one or more decimal characters,
followed by a period,
followed by an text (including nothing).

In the program, if I find this pattern I print "got a match", otherwise I print "no match".

My Java multiline pattern (regex) example source code

Given that background, here’s the source code for my Java multiline pattern matching program:

public class StringMatcherTest
{
  public static void main(String[] args)
  {
    if (args.length<1)
    {
      System.err.println("usage: java this 'testRegex'");
      System.exit(1);
    }
    String regexString = args[0];
    String spamString = "visit our site\n at http://10.1.1.1/. \nmore
poop here.";

    Pattern aPattern =
Pattern.compile(regexString,Pattern.MULTILINE);
    Matcher aMatcher = aPattern.matcher(spamString);
    if (aMatcher.find())
    {
      System.out.println("got a match");
    }
    else
    {
      System.out.println("no match");
    }
  }
}