Here’s a simple example that gets you started. It allows you to search a pattern (-cpat) in a set of files that match a certain regular expression (-fpat). You can specify a directory from where it should start searching with the option -root. -R tells the grepper to recurse through subdirectories. Below are a couple examples.

Grep.java:

import java.util.regex.*;
import java.io.*;
 
public class Grep
{
   Pattern pattern;
   FileFilter fileFilter;
 
   public Grep(String contentRegex) {
      pattern = Pattern.compile(contentRegex);
   }
 
   public void searchInDir(String startDir, String dirRegex, boolean recurse) 
                                                 throws IOException {
      final Pattern patternFile = Pattern.compile(dirRegex);
      fileFilter = new FileFilter() {
         public boolean accept(File f) {
            if (f.isDirectory()) return true;
 
            Matcher m = patternFile.matcher(f.getName());
            if (m.matches()) return true;
            return false;
         }
      };
 
      searchInDir_aux(new File(startDir), recurse);
   }
    
   public void searchInDir_aux(File dir, boolean recurse) throws IOException {
      File[] contents = dir.listFiles(fileFilter);
      for (int i=0; i<contents.length; i++) {
         File fileToCheck = contents[i];
         if (fileToCheck.isFile()) {
            searchInFile(fileToCheck);
         }
         else if (recurse) {
            searchInDir_aux(fileToCheck, recurse);
         }
      }            
   } 
 
   public void searchInFile(File file) throws IOException {
      System.out.println("=====> " + file);
      BufferedReader br = new BufferedReader(new FileReader(file));
      String line = null;
      int count=0;
      while ((line = br.readLine()) != null) {
         count++;
         Matcher m = pattern.matcher(line);
         while (m.find()) {
            System.out.println(count + ": " + line);
            System.out.print(count + ": ");
            indent(m.start());
            System.out.print("^");
            indent(m.end()-m.start()-2);
            System.out.println("^");
         }
      } 
      br.close();
   }
 
   public static void indent(int h) {
      for (int i=0; i<h; i++) 
         System.out.print(' ');
   }
 
   public static void main(String []args) throws Exception {
      String contentPattern=null, filePattern=null, startDir=null;
      boolean recurse = false;
  
      int i = -1;
      while (++i < args.length) {
         String arg = args[i];
	 if (!arg.startsWith("-")) {
            System.err.println("argument error: " + arg + "n");
            printUsage();
            System.exit(1);
         }  
         if (arg.equals("-fpat")) {
            if (i == args.length-1 || args[i+1].startsWith("-")) {
               System.err.println("Missing filename RE pattern");
               printUsage();
               System.exit(1);
            }
            filePattern = args[++i];
            continue;
         }
         else if (arg.equals("-cpat")) {
            if (i == args.length-1 || args[i+1].startsWith("-")) {
               System.err.println("Missing contents RE pattern");
               printUsage();
               System.exit(1);
            }
            contentPattern = args[++i];
            continue;
         }
         else if (arg.equals("-R")) {
            recurse = true;
            continue;
         }
         else if (arg.equals("-root")) {
            if (i == args.length-1 || args[i+1].startsWith("-")) {
               System.err.println("Missing starting directory");
               printUsage();
               System.exit(1);
            }
            startDir = args[++i];
            continue;
         }
      }                  
      if (contentPattern == null || filePattern == null) {
         printUsage();
         System.exit(1);
      }
      if (startDir == null) {
         startDir = ".";          // take current directory
      }
 
      Grep grep = new Grep(contentPattern);
      grep.searchInDir(startDir, filePattern, recurse);
   }
 
   private static void printUsage() {
      System.err.println("Usage: java [optional] -fpat fileRegex -cpat contentRegex");
      System.err.println("Where [optional] is one of the following:");
      System.err.println("    -R  recursively search subdirectories");
      System.err.println("    -root <STARTDIR>  specifies starting directory");
      System.exit(1);
   }
}

For example:

(searches all files for the pattern import)
  
C:myregextest> c:jdk1.4binjava Grep -fpat .* -cpat import
=====> .Grep$1.class
=====> .Grep.class
=====> .Grep.java
1: import java.util.regex.*;
1: ^    ^
2: import java.io.*;
2: ^    ^
 
(recursively searches c:winnt for files ending on .ini for the pattern micro.{5})
 
C:myregextest> c:jdk1.4binjava Grep -R -root c:winnt -fpat .*.ini -cpat micro.{5}
 
. . . [result]