Why does this regular expression "freeze" the program in VC++? - c++

Question

I have the following code, originally programmed using C++11's regular expressions library (#include <regex>) but now using Boost in an attempt to troubleshoot:
boost::regex reg(R"(.*?((?:[a-z][a-z]+)).*?((?:[a-z][a-z]+)).*?((?:[a-z][a-z]*[0-9]+[a-z0-9]*)).*?((?:[a-z][a-z]+)).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z]+)).*?(\d+).*?((?:[a-z][a-z]+)))", boost::regex::icase);
boost::cmatch matches;
if (boost::regex_match(request, reg) && matches.size() > 1)
{
printf("Match found");
}
else
{
printf("No match.");
}
When executed, this code seems to "freeze" on boost::regex_match(request, reg), as if it's taking a long time to process. I waited five minutes for it to process (in case this is a processing issue) but the program state was the same.
I tested the STL's regex library version of the above code online on cpp.sh and onlinegdb, and it works flawlessly there. I then copied this code into a VC++ project, and the code freezes again:
#include <iostream>
#include <string>
#include <regex>
int main()
{
std::string request = "\\login\\\\challenge\\jRJkdflp3gvTzrwiQ3tyKSqnyppmaZog\\uniquenick\\Lament\\partnerid\\0\\response\\4767846ef255a88da9b10f7c923a1e6e\\port\\-14798\\productid\\11489\\gamename\\crysiswars\\namespaceid\\56\\sdkrevision\\3\\id\\1\\final\\";
std::regex reg(R"(.*?((?:[a-z][a-z]+)).*?((?:[a-z][a-z]+)).*?((?:[a-z][a-z]*[0-9]+[a-z0-9]*)).*?((?:[a-z][a-z]+)).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z]+)).*?(\d+).*?((?:[a-z][a-z]+)))", std::regex::icase);
std::smatch matches;
if (std::regex_search(request, matches, reg) && matches.size() > 1)
{
printf("Match found");
}
else
{
printf("No match.");
}
}
The string concerned is the following:
\login\challenge\jRJwdflp3gvTrrwiQ3tyKSqnyppmaZog\uniquenick\User\partnerid\0\response\4767846ef255a83da9b10f7f923a1e6e\port-14798\productid\11489\gamename\crysiswars\namespaceid\56\sdkrevision\3\id\1\final\
I tested the same code on a Visual Studio 2017 installation on another computer (brand new project) and get the exact same result... which seems to indicate that something that the compiler is doing is causing the code to freeze/take a long time processing. I am unable to test on another compiler locally at present.
The regular expression string checks out on regex101, so functionally the expression is OK.
This is with Visual Studio 2017 Professional targeting v141.
Why is this happening, and how can I fix it?

Answer 1

Your problem is one of backtracking.
In the boost sample, you use regex_match which forces a match on the
whole string.
You will get the same result if using regex_search and adding ^..$.
However, your string can never match because you have forced it to end
on a letter, but the string really ends with a backslash .
This forces the engine to retry all those .*? positions.
The fix is to put a final .*? at the end of your regex which will let
the regex fulfill it's mission of matching the entire string.
Other things may help, you could clean up your regex a bit and/or add some
atomic groups and/or add some slashes in place of those .*?
Anyway, use this :
^.*?((?:[a-z][a-z]+)).*?((?:[a-z][a-z]+)).*?((?:[a-z][a-z]*[0-9]+[a-z0-9]*)).*?((?:[a-z][a-z]+)).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z0-9_]*)).*?(\d+).*?((?:[a-z][a-z]+)).*?(\d+).*?((?:[a-z][a-z]+)).*?$
Output
** Grp 0 - ( pos 0 : len 207 )
\login\challenge\jRJwdflp3gvTrrwiQ3tyKSqnyppmaZog\uniquenick\User\partnerid\0\response\4767846ef255a83da9b10f7f923a1e6e\port-14798\productid\11489\gamename\crysiswars\namespaceid\56\sdkrevision\3\id\1\final\
** Grp 1 - ( pos 1 : len 5 )
login
** Grp 2 - ( pos 7 : len 9 )
challenge
** Grp 3 - ( pos 17 : len 32 )
jRJwdflp3gvTrrwiQ3tyKSqnyppmaZog
** Grp 4 - ( pos 50 : len 10 )
uniquenick
** Grp 5 - ( pos 61 : len 4 )
User
** Grp 6 - ( pos 66 : len 9 )
partnerid
** Grp 7 - ( pos 76 : len 1 )
0
** Grp 8 - ( pos 78 : len 8 )
response
** Grp 9 - ( pos 94 : len 25 )
ef255a83da9b10f7f923a1e6e
** Grp 10 - ( pos 120 : len 4 )
port
** Grp 11 - ( pos 125 : len 5 )
14798
** Grp 12 - ( pos 131 : len 9 )
productid
** Grp 13 - ( pos 141 : len 5 )
11489
** Grp 14 - ( pos 147 : len 8 )
gamename
** Grp 15 - ( pos 156 : len 10 )
crysiswars
** Grp 16 - ( pos 167 : len 11 )
namespaceid
** Grp 17 - ( pos 179 : len 2 )
56
** Grp 18 - ( pos 182 : len 11 )
sdkrevision
** Grp 19 - ( pos 194 : len 1 )
3
** Grp 20 - ( pos 196 : len 2 )
id
** Grp 21 - ( pos 199 : len 1 )
1
** Grp 22 - ( pos 201 : len 5 )
final