Java RegEx check if 8th character of line is A

Question

I have a file with thousands of records, I need to filter them based on the 8th character of each line. In my case, if the 8th character is [a or A] I want to extract that line and save to a new file.

I have just put together a simple java application with 3 item 2 of which have the data I want "1st and 3rd", and I am print to console but my matcher isn't working.

my Code Example:

import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class ValidateDemo {
   public static void main(String[] args) {
      String pattern = "^.{7}([aA]{1})";

      // Create a Pattern object
      Pattern p = Pattern.compile(pattern);

      List<String> input = new ArrayList<String>();
      input.add("CARHALAALondon                     GB                         W");
      input.add("T(U   LRFonhai                     CN                         E");
      input.add("A$F   LAMuguni                     VE                         E");

      for (String ssn : input) {
         System.out.println(p + " -> " +ssn);
         if (p.matcher(ssn).matches()){
            System.out.println("Match: " + ssn);
         }

      }
  }
}

Output:

^.{7}([aA]{1}) -> CARHALAALondon                     GB  United Kingdom                       W
^.{7}([aA]{1}) -> T(U   LRFonhai                     CN  China                                E
^.{7}([aA]{1}) -> A$F   LAMuguni                     VE  Venezuela                            E

As you can see it only prints out the first SYSO, anyone any idea how I can achieve what I'm trying to do.

Thanks

G

Answer 1

You are almost there - Matcher::matches attempts to match the whole string .

This pattern should do what you want:

String pattern = "^.{7}[aA].*";

Alternatively (simpler and more efficient):

for (String ssn : input) {
   char eighth = ssn.charAt(7);
   if (eighth == 'a' || eighth == 'A') {
      System.out.println("Match: " + ssn);
   }
}    

Answer 2

I would ditch the regular expression stuff and just do a check using String's charAt(int) method as I've done in the eighthCharIsACharAt method below:

import java.util.ArrayList;
import java.util.List;
import java.util.regex.Pattern;

public class ValidateDemo {
    private static boolean eighthCharIsACharAt(String s) {
        char eighthChar = s.charAt(7);
        return (eighthChar == 'a' || eighthChar == 'A');
    }

    private static boolean eighthCharIsAMatcher(String s, Pattern p) {
        return p.matcher(s).matches();
    }

    public static void main(String[] args) {
        String pattern = "^.{7}[aA].*";
        Pattern p = Pattern.compile(pattern);

        List<String> input = new ArrayList<String>();
        input.add("CARHALAALondon                     GB                         W");
        input.add("T(U   LRFonhai                     CN                         E");
        input.add("A$F   LAMuguni                     VE                         E");

        int numIterations = 10000;

        long startTime = System.currentTimeMillis();

        for (int i = 0; i < numIterations; i++) {
            for (String s: input) {
                if (eighthCharIsAMatcher(s, p)) {
                    //System.out.println(s);
                }
            }
        }

        System.out.println("Matcher elapsed time: " + (System.currentTimeMillis() - startTime) + " ms");

        startTime = System.currentTimeMillis();

        for (int i = 0; i < numIterations; i++) {
            for (String s: input) {
                if (eighthCharIsACharAt(s)) {
                    //System.out.println(s);
                }
            }
        }

        System.out.println("charAt elapsed time: " + (System.currentTimeMillis() - startTime) + " ms");
    }
}

Regular expressions are great, but not very efficient when used in a loop. In your specific case, it seems like overkill.

In my test comparison using charAt versus Pattern matches, charAt wins by over a factor of 10.

Run output:

Matcher elapsed time: 64 ms
charAt elapsed time: 4 ms