In this example, the format of each line is verified against a regular expression.

Note that a String literal representing a regular expression needs to escape all backslashes. In effect, all of the backslashes are simply doubled.

Example

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

public class LineMatcher {

  /**
  * Verifies that each line of a text file starts with:
  *   \N + tab + integer + tab + text
  * where "text" contains word characters (\w).
  *
  * The corresponding regular expression is:
  *   "^\\N\t(\d)+\t(\w)+"
  *
  * The String passed to Pattern needs to double every backslash:
  *   "^\\\\N\\t(\\d)+\\t(\\w)+"
  *
  * If a line is not of the expected pattern, then an
  * IllegalStateException is thrown.
  */
  public static void findBadLines( final File aFile ) {
    //...checks on aFile are elided

    //Pattern and Matcher are used here, not String.matches(regexp),
    //since String.matches(regexp) would repeatedly compile the same
    //regular expression
    Pattern regexp = Pattern.compile("^\\\\N\\t(\\d)+\\t(\\w)+");
    Matcher matcher = regexp.matcher("");

    LineNumberReader lineReader = null;
    try {
      lineReader = new LineNumberReader( new FileReader(aFile) );
      String line = null;
      while ((line = lineReader.readLine()) != null){
        matcher.reset( line ); //reset the input
        if ( !matcher.find() ) {
          String msg = "Line " + lineReader.getLineNumber() + " is bad: " + line;
          throw new IllegalStateException(msg);
        }
      }
    }
    catch (FileNotFoundException ex) {
      ex.printStackTrace();
    }
    catch (IOException ex){
      ex.printStackTrace();
    }
    finally {
      try {
        if (lineReader!= null) lineReader.close();
      }
      catch (IOException ex) {
        ex.printStackTrace();
      }
    }
  }

  /**
  * Test harness.
  */
  public static void main (String... arguments) {
    final File file = new File( "C:\\Temp\\RegexpTest.txt" );
    LineMatcher.findBadLines( file );
  }
}