Be wary of the these convenience methods, which compile a regular expression each time they are called:

• Pattern.matches
• String.matches
• String.replaceAll
• String.replaceFirst
• String.split

Here's a comparison of the performance of String.matches versus the use of Pattern and Matcher.

An example run (with the HotSpot compiler turned off) gives:

Time for String: 67.693 ms
Time for Pattern: 12.178 ms

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/** JDK 7+. */
public final class FindMatch {

  /**
   Count the number of times the word "assert" occurs in a file.
  
   Compare the execution times of two techniques.
  */
  public static void main (String... arguments) throws IOException{
    FindMatch findMatch = new FindMatch("assert");
    Path path = Paths.get("C:\\Temp\\InheritanceInterpreter.java");

    Stopwatch stopwatch = new Stopwatch();
    stopwatch.start();
    findMatch.countMatchesUsingString(path);
    stopwatch.stop();
    log("Time for String: " + stopwatch);

    stopwatch.start();
    findMatch.countMatchesUsingPattern(path);
    stopwatch.stop();
    log("Time for Pattern: " + stopwatch);
  }

  public FindMatch(String target){
    this.targetWord = target;
    this.pattern = Pattern.compile(targetWord);
  }

  int countMatchesUsingString(Path path) throws IOException {
    int result = 0;
    try (Scanner scanner =  new Scanner(path, ENCODING.name())){
      while (scanner.hasNextLine()){
        String line = scanner.nextLine();
        if( line.matches(targetWord) ){
          ++ result;
        }
      }      
    }
    return result;
  }
  
  int countMatchesUsingPattern(Path path) throws IOException {
    int result = 0;
    Matcher matcher = pattern.matcher("");
    try (Scanner scanner =  new Scanner(path, ENCODING.name())){
      while (scanner.hasNextLine()){
        String line = scanner.nextLine();
        matcher.reset(line); //reset the input each time
        if (! matcher.find()) {
          ++result;
        }
      }      
    }
    return result;
  }
  
  // PRIVATE 
  private final String targetWord;
  private final Pattern pattern;
  private final static Charset ENCODING = StandardCharsets.UTF_8;

  private static void log(Object thing){
    System.out.println(thing);
  }
}