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POSlower than expected Java regex performance
 

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    <p>I have been tasked with reading large CSV files (300k+ records) and apply regexp patterns to each record. I have always been a PHP developer and never really tried any other languages, but decided I should take the dive and attempt to do this with Java which I assumed would be much faster.</p> <p>In fact, just reading the CSV file line by line was 3x faster in Java. However, when I applied the regexp requirements, the Java implementation proved to take 10-20% longer than the PHP script. </p> <p>It is very well possible that I did something wrong in Java, because I just learned this as I went today. Below are the two scripts, any advice would be greatly appreciated. I really would like to not give up on Java for this particular project.</p> <p>PHP CODE</p> <pre><code>&lt;?php $bgtime=time(); $patterns =array( "/SOME REGEXP/", "/SOME REGEXP/", "/SOME REGEXP/", "/SOME REGEXP/" ); $fh = fopen('largeCSV.txt','r'); while($currentLineString = fgetcsv($fh, 10000, ",")) { foreach($patterns AS $pattern) { preg_match_all($pattern, $currentLineString[6], $matches); } } fclose($fh); print "Execution Time: ".(time()-$bgtime); ?&gt; </code></pre> <p>JAVA CODE</p> <pre><code>import au.com.bytecode.opencsv.CSVReader; import java.io.FileReader; import java.io.IOException; import java.util.regex.Pattern; import java.util.regex.Matcher; import java.util.ArrayList; public class testParser { public static void main(String[] args) { long start = System.currentTimeMillis(); String[] rawPatterns = { "SOME REGEXP", "SOME REGEXP", "SOME REGEXP", "SOME REGEXP" }; ArrayList&lt;Pattern&gt; compiledPatternList = new ArrayList&lt;Pattern&gt;(); for(String patternString : rawPatterns) { Pattern compiledPattern = Pattern.compile(patternString); compiledPatternList.add(compiledPattern); } try{ String fileName="largeCSV.txt"; CSVReader reader = new CSVReader(new FileReader(fileName)); String[] header = reader.readNext(); String[] nextLine; String description; while( (nextLine = reader.readNext()) != null) { description = nextLine[6]; for(Pattern compiledPattern : compiledPatternList) { Matcher m = compiledPattern.matcher(description); while(m.find()) { //System.out.println(m.group(0)); } } } } catch(IOException ioe) { System.out.println("Blah!"); } long end = System.currentTimeMillis(); System.out.println("Execution time was "+((end-start)/1000)+" seconds."); } } </code></pre>
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    1. CONot entirely related to your regex problem, but you might want to look at http://download.oracle.com/javase/6/docs/api/java/util/Scanner.html. You might find your CSVReader class is not needed. Not creating all of those temporary Strings (`nextLine` appears to have at least 7 Strings, but you only need one) might improve performance.
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