std regex_search to match only current line - c++

Question

I use a various regexes to parse a C source file, line by line. First i read all the content of file in a string:
ifstream file_stream("commented.cpp",ifstream::binary);
std::string txt((std::istreambuf_iterator<char>(file_stream)),
std::istreambuf_iterator<char>());
Then i use a set of regex, which should be applied continusly until the match found, here i will give only one for example:
vector<regex> rules = { regex("^//[^\n]*$") };
char * search =(char*)txt.c_str();
int position = 0, length = 0;
for (int i = 0; i < rules.size(); i++) {
cmatch match;
if (regex_search(search + position, match, rules[i],regex_constants::match_not_bol | regex_constants::match_not_eol))
{
position += ( match.position() + match.length() );
}
}
But it don't work. It will match the comment not in the current line, but it will search whole string, for the first match, regex_constants::match_not_bol and regex_constants::match_not_eol should make the regex_search to recognize ^$ as start/end of line only, not end start/end of whole block. So here is my file:
commented.cpp:
#include <stdio.h>
//comment
The code should fail, my logic is with those options to regex_search, the match should fail, because it should search for pattern in the first line:
#include <stdio.h>
But instead it searches whole string, and immideatly finds //comment. I need help, to make regex_search match only in current line. The options match_not_bol and match_not_eol do not help me. Of course i can read a file line by line in a vector, and then do match of all rules on each string in vector, but it is very slow, i have done that, and it take too long time to parse a big file like that, that's why i want to let regex deal with new lines, and use positioning counter.

Answer 1

If it is not what you want please comment so I will delete the answer
What you are doing is not a correct way of using a regex library.
Thus here is my suggestion for anyone that wants to use std::regex library.
It only supports ECMAScript that somehow is a little
poor than all modern regex library.
It has bugs as many as you like ( just I found ):
the same regex but different results on Linux and Windows only C++
std::regex and ignoring flags
std::regex_match and lazy quantifier with strange behavior
In some cases (I test specifically with std::match_results ) It is 200 times slower in comparison to std.regex in d language
It has very confusing flag-match and almost it does not work (at least for me)
conclusion: do not use it at all.
But if anyone still demands to use c++ anyway then you can:
use boost::regex about Boost library because:
It is PCRE support
It has less bug ( I have not seen any )
It is smaller in bin file ( I mean executable file after compiling )
It is faster then std::regex
use gcc version 7.1.0 and NOT below. The last bug I found is in version 6.3.0
use clang version 3 or above
If you have enticed (= persuade) to NOT use c++ then you can use:
Use d regular expression link library for large task: std.regex and why:
Fast Faster Command Line Tools in D
Easy
Flexible drn
Use native pcre or pcre2 link that have been written in c
Extremely fast but a little complicated
Use perl for a simple task and specially Perl one-liner link

Answer 2

#include <stdio.h>
//comment
The code should fail, my logic is with those options to regex_search, the match should fail, because it should search for pattern in the first line:
#include <stdio.h>
But instead it searches whole string, and immideatly finds //comment. I need help, to make regex_search match only in current line.
Are you trying to match all // comments in a source code file, or only the first line?
The former can be done like this:
#include <iostream>
#include <fstream>
#include <regex>
int main()
{
auto input = std::ifstream{"stream_union.h"};
for(auto line = std::string{}; getline(input, line); )
{
auto submatch = std::smatch{};
auto pattern = std::regex(R"(//)");
std::regex_search(line, submatch, pattern);
auto match = submatch.str(0);
if(match.empty()) continue;
std::cout << line << std::endl;
}
std::cout << std::endl;
return EXIT_SUCCESS;
}
And the later can be done like this:
#include <iostream>
#include <fstream>
#include <regex>
int main()
{
auto input = std::ifstream{"stream_union.h"};
auto line = std::string{};
getline(input, line);
auto submatch = std::smatch{};
auto pattern = std::regex(R"(//)");
std::regex_search(line, submatch, pattern);
auto match = submatch.str(0);
if(match.empty()) { return EXIT_FAILURE; }
std::cout << line << std::endl;
return EXIT_SUCCESS;
}
If for any reason you're trying to get the position of the match, tellg() will do that for you.