c# - Benchmarking .Net regex options - or what does RightToLeft do? -

i've created demo application test performance of regexes. third test uses option righttoleft.

it seems speeds process lot! why? do?

here test app:

using system; using system.collections.generic; using system.diagnostics; using system.linq; using system.text; using system.text.regularexpressions; using system.threading.tasks;  namespace consoleapplication1 {     class program     {         private const string isrequestfordirectorywithouttrailingslashregex = @"^(?#path:)(.*/)?(?#lastpart:)(?<!\.asmx|\.aspx/)([^./?#]+?)(?#querystring:)(\?.*?)(?#anchor:)?(#.*)?$";          private static string[] tests = new string[] {             "http://localhost/manager/page.aspx",             "http://localhost/manager/",             "http://localhost/manager",             "http://localhost/manager/?param=value",             "http://localhost/manager/dir?param=value"         };          static void main(string[] args)         {             test1();             test2();             test3();             test4();              console.writeline();             console.readline();         }          public static void test1()         {             regex regex = new regex(isrequestfordirectorywithouttrailingslashregex);             dowork("1", regex);         }          public static void test2()         {             regex regex = new regex(isrequestfordirectorywithouttrailingslashregex, regexoptions.compiled);             dowork("2", regex);         }          public static void test3()         {             regex regex = new regex(isrequestfordirectorywithouttrailingslashregex, regexoptions.compiled | regexoptions.righttoleft);             dowork("3", regex);         }          public static void test4()         {             regex regex = new regex(isrequestfordirectorywithouttrailingslashregex, regexoptions.compiled | regexoptions.righttoleft | regexoptions.ignorecase);             dowork("4", regex);         }         static void dowork(string name, regex regex)         {             stopwatch sp = new stopwatch();             sp.start();              (int = 0; < 100000; i++)             {                 foreach (string s in tests)                 {                     regex.ismatch(s);                 }             }              foreach (string s in tests)             {                 console.writeline(":" + s + ":" + regex.ismatch(s).tostring());             }              sp.stop();              console.writeline("test " + name + ": " + sp.elapsedticks);         }     } } 

the regexoptions.righttoleft can useful when trying match pattern expect find @ end of input string, because documentation say: search goes right left left right starting @ final character in input string, regex though still applied left right.

your regex seems looking trailing slash directory paths, seems situation fits description.

although expression looking trailing slash, presence of 2 anchors (^ , $) makes reasoning wrong, because regex can match in 1 possible way no matter starts.

i going keep looking actual reason behind this, leave answer is.

on other hand, .*/ part of expression right after (?#path:) part @ start of expression consumes entire input string goes recursively each time find last /, when starting search further ahead there might not lot of backtracking.