Why exception with this regular expression pattern (tr1::regex)?

Question

Hi everyone,

I came accross a very strange problem with tr1::regex (VS2008) that I can't figure out the reason for. The code at the end of the post compiles fine but throws an exception when reaching the 4th regular expression definition during execution:

Microsoft C++ exception: std::tr1::regex_error at memory location 0x0012f5f4..

However, the only difference I can see (maybe I am blind) between the 3rd and 4th one is the 'NumberOfComponents' instead of 'SchemeVersion'. At first I thought maybe both (3rd and 4th) are wrong and the error from the 3rd is just triggered in the 4th. That seems not to be the case as I moved both of them around and put multiple other regex definitions between them. The line in question always triggers the exception.

Does anyone have any idea why that line

std::tr1::regex rxNumberOfComponents("\\NumberOfComponents:(\\s*\\d+){1}");

triggers an exception but

std::tr1::regex rxSchemeVersion("\\SchemeVersion:(\\s*\\d+){1}");

doesn't? Is the runtime just messing with me?

Thanks for the time to read this and for any insights.
T

PS: I am totally sure the solution is so easy I have to hit my head against the nearest wall to even out the 'stupid question' karma ...

#include <regex>

int main(void)
{
   std::tr1::regex rxSepFileIdent("Scanner Separation Configuration");
   std::tr1::regex rxScannerNameIdent("\\ScannerName:((\\s*\\w+)+)");

   std::tr1::regex rxSchemeVersion("\\SchemeVersion:(\\s*\\d+){1}");
   std::tr1::regex rxNumberOfComponents("\\NumberOfComponents:(\\s*\\d+){1}");

   std::tr1::regex rxConfigStartIdent("Configuration Start");
   std::tr1::regex rxConfigEndIdent("Configuration End");

   return 0;
}

Answer 1

You need to double-escape your backslashes - once for the regex itself, a second time for the string they're in.

The one that starts with S works because \S is a valid regex escape (non-whitespace characters). The one that starts with N does not (because \N is not a valid regex escape).

Instead, use "\\\\SchemeVersion: et cetera.