Hello guys,

I'm totally confused... I tried to benchmark two regexes /^(b|a)*$/ and /^(a+|b)*/ against input string "aaaaaaaaab" x <<some big numbers>>. I was sure that the second one is much faster for such string, but when I run my script the results were quite shocking to me (I use similar approach to J. Friedl in his book):

<no of chars x 10 performed 1000 times>;<time in sec - 1st regex>;<tim
+e in sec - 2st regex>
200;0.113;0.060
400;0.225;0.112
800;0.448;0.226
1600;0.890;0.763
3200;1.769;1.627
6400;3.539;3.427
12800;7.120;7.215
25600;14.256;58.209
51200;29.847;95.463
102400;65.144;148.634
[download]

I don't understand two things. First, why for strings shorter than 128000 chars times are almost the same (IMO the 2nd regex should be faster). Second, why for string larger than 256000 chars the 2nd regex is much slower than the 1st one?

I looked in debug optimizer information, but I didn't find anything particularly interesting (i.e. there are no applicable optimizations for this input string).

It has caused me a headache so I decided to check the same cases in Java. And it seems that regex engine in Java works as I expected:

200;0.145;0.125
400;0.217;0.081
800;0.436;0.165
1600;0.854;0.334
3200;1.83;0.661
6400;4.185;1.327
12800;8.488;2.767
25600;17.791;6.181
51200;37.271;12.856
102400;76.931;24.81
[download]

I have no idea why Perl regex engine behaves strangely in this case. If someone could explain it to me I would be grateful.

Perl testing module:

use Time::HiRes 'time';
#~ use re 'debug';

$TimesToDo = 1000;
$NoOfChars = 100;
$Multiplier = 2;
$NoOfExec = 10;
$InputString = "aaaaaaaaab";

for($i = 0; $i < $NoOfExec; $i++){
    $NoOfChars = $NoOfChars * $Multiplier;
    $TestString = $InputString x $NoOfChars;
    $Count = $TimesToDo;
    $StartTime = time();
    while ($Count-- > 0) {
        $TestString =~ m/^(b|a)*$/;
    }
    $EndTime = time();
    $Diff = $EndTime - $StartTime;

    $Count = $TimesToDo;
    $StartTime = time();
    while ($Count-- > 0) {
        $TestString =~ m/^(a+|b)*$/;
    }
    $EndTime = time();
    printf("%d;%.3f;%.3f\n", $NoOfChars, $Diff, $EndTime - $StartTime)
+;
}
[download]

Java testing class (it can be executed using command "javac test2.java && java -Xss512m test2"):

import java.util.regex.*;
public class test2 {
    public static void main(String [] args)
    {
        Matcher regex1 = Pattern.compile("^(b|a)*$").matcher("");
        Matcher regex2 = Pattern.compile("^(a+|b)*$").matcher("");
        int timesToDo = 1000;
        int noOfChars = 100;
        int multiplier = 2;
        int noOfExec = 10;
        String inputString = "aaaaaaaaab";

        for(int j = 0; j < noOfExec; j++){
            StringBuffer temp = new StringBuffer();
            noOfChars = noOfChars * multiplier;
            for (int i = 0; i < noOfChars; i++)
                temp.append(inputString);
            String testString = temp.toString();

            long count = timesToDo;
            long startTime = System.currentTimeMillis();
            while (--count > 0)
                regex1.reset(testString).find();
            double diffSeconds = (System.currentTimeMillis() - startTi
+me)/1000.0;

            count = timesToDo;
            startTime = System.currentTimeMillis();
            while (--count > 0)
                regex2.reset(testString).find();
            double seconds = (System.currentTimeMillis() - startTime)/
+1000.0;
            System.out.println(noOfChars+";"+diffSeconds+";"+seconds);
        }
    }
}
[download]

Btw. I use Manjaro Linux with Perl 5.20.1.

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