Grep.java

Here is a program that reads through the lines of a file one by one, looking in each line for a user-specified regular expression. (This is similar to the Unix command grep.)

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

public class Grep 
{
  public static void main (String [] args) throws IOException
  {
    String line;
    int lines = 0;
    int matches = 0;

    // Check the command line, prompt user if needed   
    if ( args.length != 2 )
    {
      System.out.print("java Grep pattern fileName\n");
      System.exit( 1 );
    }
    System.out.print  ("Find " + args[0]);
    System.out.println(" in "  + args[1]);

    // Create a Scanner for the file
    File in = new File(args[1]);
    Scanner scan = new Scanner( in );
    
    // Get a regular expression from the command line  
    Pattern pat = Pattern.compile( args[0] );

    // Match lines against the Regular Expression
    while ( scan.hasNextLine() ) 
    {
      line = scan.nextLine();
      lines++;
      
      // Check if the current line contains the pattern
      Matcher match = pat.matcher( line );
      if (match.find()) 
      {
        System.out.println ("" + lines + ": " + line);
        matches++;
      }
    }
    
    System.out.println( lines + " lines, " + matches + " matches" );
  }
}