diff --git a/musrSim.cc b/musrSim.cc
index de5fb49..050c61d 100644
--- a/musrSim.cc
+++ b/musrSim.cc
@@ -1,3 +1,4 @@
+#include <regex>
 #include "musrDetectorConstruction.hh"
 #include "musrPhysicsList.hh"
 #include "musrPrimaryGeneratorAction.hh"
@@ -76,9 +77,13 @@ int main(int argc,char** argv) {
 
   // UserInitialization classes (mandatory)
   //  musrDetectorConstruction* musrdetector = new musrDetectorConstruction;
+  //Allow for argv[1] generic paths and only extract the runnumber before .mac
   if (argc>1) {
-    G4int myRunNr=atoi(argv[1]);       // Get the run number from the name of the 
-                                       // parameter file, if it starts with a number.
+    std::regex pattern(R"((\d+)(?=(?=[^/]*\.mac$)))"); // for  an input like /some/path987/1234.mac for argv[1]. Matches last number (1234) before ".mac"
+    std::smatch match;
+    G4int myRunNr = 0;
+    std::string argv1str(argv[1]);
+    if (std::regex_search(argv1str, match, pattern)) myRunNr = G4int(std::stoi(match.str(1)));
     if (myRunNr>0)  {runManager->SetRunIDCounter(myRunNr);}
     //    musrdetector->SetInputParameterFileName(argv[1]);
   }