Java Practices -> Compile regular expressions once

Compile regular expressions once

Pattern objects compile the regular expressions which are passed to them. (The compilation happens in memory.) If a regular expression is used many times, then this compilation should be performed only once.

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

Example

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

An example run gives :

Time for String: 70 ms
Time for Pattern: 10 ms


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

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... aArguments){
    FindMatch findMatch = new FindMatch("assert");
    File file = new File("C:\\Temp\\InheritanceInterpreter.java");

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

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

  public FindMatch(String aTarget){
    fTargetWord = aTarget;
    fPattern = Pattern.compile(fTargetWord);
  }

  public int countMatchesUsingString(File aFile){
    int result = 0;
    try {
      Scanner scanner = new Scanner(aFile);
      while ( scanner.hasNextLine() ){
        String line = scanner.nextLine();
        if( line.matches(fTargetWord) ){
          ++ result;
        }
      }
    }
    catch (FileNotFoundException ex) {
      log(ex);
    }
    return result;
  }

  public int countMatchesUsingPattern(File aFile) {
    int result = 0;
    Matcher matcher = fPattern.matcher("");
    try {
      Scanner scanner = new Scanner(aFile);
      while ( scanner.hasNextLine() ){
        String line = scanner.nextLine();
        matcher.reset( line ); //reset the input each time
        if ( ! matcher.find() ) {
          ++result;
        }
      }
    }
    catch (FileNotFoundException ex) {
      log(ex);
    }
    return result;
  }

  // PRIVATE //
  private final String fTargetWord;
  private final Pattern fPattern;

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