I'm trying to utilize the 'TR1' regular expression extensions for some C++ string parsing.
I've read that the <regex> header and namespace std::tr1 are required for this
I can compile with the <regex> header present(though it forces me to use either the flag, -std=c++0x or -std=gnu++0x)
However, when I attempt to use the std::tr1 namespace in my program, compiling fails with message that tr1 "is not a namespace name". I cant do things like,
std::tr1::regex rx("mypattern");
I've read that TR1 regular expressions have been supported since gcc 4.3.0. I'm using g++ through gcc 4.4.5.
Am I missing something?
g++ 4.7 doesn't implement regular expressions yet.
But despite that fact, in C++11 regex has been moved from the namespace std::tr1 to std. So, instead of std::tr1::regex, you should write std::regex:
std::regex rx("mypattern");
I don't know for which g++ versions before 4.7 this applies, too. But this ideone example compiles fine with g++ 4.7. However, remember that the regex implementation isn't implemented in this compiler version.
Related
I recently found out that regex support in gcc 4.8 is incomplete, and it was truly implemented in gcc 4.9 (see Is gcc 4.8 or earlier buggy about regular expressions?).
So, wanting to work with regex in my c++ program, I updated my gcc to 4.9 following this instructions (https://askubuntu.com/questions/466651/how-do-i-use-the-latest-gcc-4-9-on-ubuntu-14-04).
Now when I try to compile my program it says that in order to #include <regex> I have to specify the compiler flag -std=c++11, which I did, and now I'm faced with new compilation problems that I didn' had before (‘constexpr’ needed for in-class initialization of static data member).
Given that, I think for now it is best to stick to gcc 4.8 and not specify the gnu++11 flag in compilation. Back to square 1.
So, can I work with regular expressions in c++ if I do not want to switch to gcc 4.9 nor flag the compiler with c++11? Is there another way?
Thanks!
PS: actually it's the c++11 flag that causes the compilation issues, not the version of gcc, right?
You can install the PCRE library and use that instead of the C++11 standard regular expressions. PCRE is really designed as a C library/interface, rather than C++, but writing a couple of trivial wrapper classes or just using it as a C library is quite easy.
The error most likely means you were relying on a non-standard GCC extension to initialize a non-integer type like this:
struct X {
static const double d = 3.14;
};
That was not valid in C++98, but was supported by GCC.
The C++11 standard added support for initializing non-integer types like that, but you need to use constexpr e.g.
struct X {
static constexpr double d = 3.14;
};
When you compile with -std=c++11 or -std=gnu++11 the old GCC-specific extension is no longer supported. and you have to use the standard C++11 way, using constexpr.
So you could easily resolve the error by changing it to constexpr, or to make it compatible with GCC's C++98 extension and also C++11:
struct X {
#if __cplusplus > 199711L
static constexpr double d = 3.14;
#else
// this is non-standard but allowed by GCC in C++98 mode
static const double d = 3.14;
#endif
};
That will allow you to compile with -std=c++11, and so you can use GCC 4.9's working std::regex.
I don't get this. GCC is supposed to support but accoriding to their
http://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.tr1
Status page "7 Regular Expressions are not supported".
But then at "28 Regular expressions" - they are checked as supported
http://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2011
Could you please explain what is actually the standard and what is not?
GCC 4.9 does indeed support the C++11 <regex> functionality but not the tr1 version. Note that the difference is that parts (all?) of the latter exist within a tr1:: namespace while the C++11 <regex> is within namespace std. There's not much point to going backwards and adding in tr1 support when C++11 has been published for some time now.
Following information can be found from GCC 4.9 release notes:
"Support for various C++14 additions have been added to the C++ Front End, on the standard C++ library side the most important addition is support for the C++11 regex"
If you want to install the latest GCC4.9 version to try by yourself you can follow below SO link:
How do I compile and run GCC 4.9.x?
Here is the sample program which has compiled using gcc4.9 and subsequent run.
//Sample Program
#include <regex>
#include <iostream>
using namespace std;
int main() {
regex reg("[0-9]+");
if (regex_match("123000", reg)) {
cout << "It's a match!" <<endl;
}
return 0;
}
$g++ -std=c++11 foo.cpp -o foo
$ g++ -v
Using built-in specs.
COLLECT_GCC=/home/mantosh/gcc-4.9.0/bin/g++
COLLECT_LTO_WRAPPER=/home/mantosh/gcc-4.9.0/libexec/gcc/x86_64-unknown-linux-gnu/4.9.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /home/mantosh/objdir/../gcc-4.9.0/configure --disable-multilib --prefix=/home/mantosh/gcc-4.9.0
Thread model: posix
gcc version 4.9.0 (GCC)
$ ./foo
It's a match!
I am using codeblocks and i can't make stoi() function work. I read other questions regarding this issue but I couldn't solve it. I checked C+11, I am using namespace std and I have the string header. I don't know how can I solve this problem.
error: 'stoi' was not declared in this scope
found the answer on almost the same question here:
"It seems your MinGW needs a patch: Enabling string conversion functions in MinGW"
From this link you can download a .zip, than follow the instructions.
This patch enables the following list of C++11 functions and
templates in the std namespace:
stoi, stol, stoul, stoll, stof, stod, stold, to_string, to_wstring
Attention, maybe you will haver errors with later versions, after using the copy/paste:
As Conduit said here:
"People should note that direct replacement of the files is not guaranteed to be safe on versions later than MinGW GCC 4.7 - use the pastebin snippets, open the files, and comment/add to the existing files. Applied as such, this still works fine on MinGW GCC 4.8.1 afai"
I would use atoi on the str.data() returned value, since it is supported in all compiler versions.
Use
#include <string>
That should work. For reference: C++11 standard plus minor editorial changes.
Or perhaps you are forgetting to prefix with std::?
A workaround is calling .c_str() and using the <cstdlib> or <stdlib.h> functions.
I'm trying to use a regular expression to accept strings that have sequences like
ifelseifelseififif
So every else needs an if, but not every if needs an else. I know I could do this with pen and paper with a simple regular expression like this ((if)*+(ifelse)*)* .Now I'm not sure if I'll be able to do this with the library as I've never used it before. So would it be possible to accept or reject a string based on a regular expression like the one I wrote above?
I wrote this sample to get my feet wet and I don't understand why it returns false. Isn't regex_search() supposed to find substring matches? That snippet prints nope every time.
#include <regex>
#include <iostream>
using namespace std;
int main(){
string sequence="ifelse";
regex rx("if");
if(regex_search(sequence.begin(),sequence.end(),rx)){
cout<<"match found"<<endl;
}else{
cout<<"nope"<<endl;
}
}
I'm using g++ 4.7 and have tried compiling with both g++ -std=gnu+11 reg.cpp and g++ -std=c++11 reg.cpp
If you are compiling with g++ it may be because regex is not fully supported yet. See here for current C++11 status in g++.
This prints "match found", I just ran it. It wouldn't compile if you weren't using c++11 but heres how I compiled it.
clang++ -std=c++11 -stdlib=libc++ reg.cpp
OK, this isn't the original program I had this problem in, but I duplicated it in a much smaller one. Very simple problem.
main.cpp:
#include <iostream>
#include <regex>
using namespace std;
int main()
{
regex r1("S");
printf("S works.\n");
regex r2(".");
printf(". works.\n");
regex r3(".+");
printf(".+ works.\n");
regex r4("[0-9]");
printf("[0-9] works.\n");
return 0;
}
Compiled successfully with this command, no error messages:
$ g++ -std=c++0x main.cpp
The last line of g++ -v, by the way, is:
gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3)
And the result when I try to run it:
$ ./a.out
S works.
. works.
.+ works.
terminate called after throwing an instance of 'std::regex_error'
what(): regex_error
Aborted
It happens the same way if I change r4 to \\s, \\w, or [a-z]. Is this a problem with the compiler? I might be able to believe that C++11's regex engine has different ways of saying "whitespace" or "word character," but square brackets not working is a stretch. Is it something that's been fixed in 4.6.2?
EDIT:
Joachim Pileborg has supplied a partial solution, using an extra regex_constants parameter to enable a syntax that supports square brackets, but neither basic, extended, awk, nor ECMAScript seem to support backslash-escaped terms like \\s, \\w, or \\t.
EDIT 2:
Using raw strings (R"(\w)" instead of "\\w") doesn't seem to work either.
Update: <regex> is now implemented and released in GCC 4.9.0
Old answer:
ECMAScript syntax accepts [0-9], \s, \w, etc, see ECMA-262 (15.10). Here's an example with boost::regex that also uses the ECMAScript syntax by default:
#include <boost/regex.hpp>
int main(int argc, char* argv[]) {
using namespace boost;
regex e("[0-9]");
return argc > 1 ? !regex_match(argv[1], e) : 2;
}
It works:
$ g++ -std=c++0x *.cc -lboost_regex && ./a.out 1
According to the C++11 standard (28.8.2) basic_regex() uses regex_constants::ECMAScript flag by default so it must understand this syntax.
Is this C++11 regex error me or the compiler?
gcc-4.6.1 doesn't support c++11 regular expressions (28.13).
The error is because creating a regex by default uses ECMAScript syntax for the expression, which doesn't support brackets. You should declare the expression with the basic or extended flag:
std::regex r4("[0-9]", std::regex_constants::basic);
Edit Seems like libstdc++ (part of GCC, and the library that handles all C++ stuff) doesn't fully implement regular expressions yet. In their status document they say that Modified ECMAScript regular expression grammar is not implemented yet.
Regex support improved between gcc 4.8.2 and 4.9.2. For example, the regex =[A-Z]{3} was failing for me with:
Regex error
After upgrading to gcc 4.9.2, it works as expected.