Παράδειγμα: Εύρεση κανονικών εκφράσεων σε αρχείο

/*
 * Globally match regular expression and print
 * Modelled after the Unix command with the same name
 * D. Spinellis, January 2004-2024
 */

import java.util.regex.*;
import java.io.*;

class Grep {
    public static void main(String args[]) {
        if (args.length != 2) {
            System.err.println("Usage: Grep pattern file");
            System.exit(1);
        }

        try (var in = new BufferedReader(new InputStreamReader(
                        new FileInputStream(args[1])))) {
            Pattern compiledRe = Pattern.compile(args[0]);
            String s;
            while ((s = in.readLine()) != null) {
                Matcher m = compiledRe.matcher(s);
                if (m.find())
                    System.out.println(s);
            }
        } catch (FileNotFoundException e) {
            System.err.println("Unable to open file " + args[1] + ": " + e.getMessage());
            System.exit(2);
        } catch (PatternSyntaxException e) {
            System.err.println("Invalid RE syntax: " + e.getDescription());
            System.exit(3);
        } catch (IOException e) {
            System.err.println("Error reading line: " + e.getMessage());
            System.exit(4);
        }
    }
}