Finding variable patterns using C++ STL Library

Question

I have a file in which I have to look for a pattern.

The pattern is like this:

"ABCD"

A, B, C, D are variables which can be 0-9 and AF, and nothing else.

Few sample string in this file is :

at+creg=2 
OK 

at+creg? 
+CREG: 2,1,"03EB","3AC7" 
+CREG: 2,1,"03FC","9AC9" 

My pattern matching code should output:

03EB, 3AC7, 03FC, 9AC9

I want to do this using a functions from STL library.

If we have two strings, such as:

std::string str ("There are two needles in this haystack with needles.");
std::string str2 ("needle");  

then we can use

str.find(str2); 

to look for the pattern needle .

But in my case I do not have a fixed string, as A,B,C,D above are variable. So kindly help me to understand how to proceed here.

Answer 1

Regular expressions do what you want. Here is an example on how they work:

#include <regex>
#include <string>

using namespace std;

int main()
{
  // regular expressions which has the format you desire to match later
  regex isHexadecimal(R"(([[:d:]]|A|B|C|D|E|F)+)");
  string text = "asdlkfj 1C3AF";
  smatch result;

  regex_search (text, result, isHexadecimal);

  for (int i=0; i < result.size(); i++) {
      //result.position(i); // here are your positions
  }
  return 1;
}

All the magic happens in this line: regex isHexadecimal(R"(([[:d:]]|A|B|C|D|E|F)+)"); The R"(textstringtext)" syntax is for raw strings (strings in which you can have special characters without having to escape them). The regex itself is ([[:d:]]|A|B|C|D|E|F)+ : [[:d:]] is for digit. | means or. A , B are single characters to be matched. + means "one or more of".

Thus, the expression will catch a sequence with one or more digit(from 0 to 9) or A or B or C or D or E or F.

Answer 2

It seems you are searching hexadecimal digits. If the case of letters is not important then you can apply standard C function std::isxdigit declared in header <cctype> . Usually C standard functions are placed in the global namespace. So you may use ::isxdigit to distinguish it from the C++ function with the same name that has two parameters. Otherwise you can wrap the function in std::function or cast the function to required type.

Here is a demonstrative program. Instead of std::find_if algorithm I used std::copy_if algorithm just for the demonstrative purpose.

#include <iostream>
#include <algorithm>
#include <string>
#include <sstream>
#include <cctype>
#include <iterator>

int main()
{
    std::istringstream is( " ABC 123AA43 Jh12 76-12 fedcba1" );

    std::copy_if ( std::istream_iterator<std::string>( is ),
                   std::istream_iterator<std::string>(),
                   std::ostream_iterator<std::string>( std::cout, "\n" ),
                   []( const std::string &s )
                   {
                       return std::all_of( std::begin( s ), std::end( s ), ::isxdigit );
                   } );

}

The output is

ABC
123AA43
fedcba1