Selectively replace (") doublequotes in a std::string in C++

Question

I want to selectively replace the (") doublequotes in a C++ std::string. i.e. i want to replace all occurences of (") doublequotes in a string except the 1st and last occurence of (") doublequotes in the string.

Example- following code replaces ALL occurence of (") doublequotes

std::string str = "\"Hello people \" how are you doing \" what are \" you upto \"";
str = std::regex_replace(str, std::regex("\\\""), """);

However, i don't want to replace the 1st and last occurenece of the string.

i.e. in the string i don't want to replace (") just before "Hello" & the one at the end.

std::string str = "\"Hello people \" how are you doing \" what are \" you upto \"";

Answer 1

A solution without regex and boost

  auto aPos1 = aString.find_first_of("\"");
  auto aPos2 = aString.find_last_of("\"");
  auto aPos = aString.length() - 1;
  for( ; aPos > aPos1 ; aPos--)
  {
    auto aVal = aString.at(aPos);
    if(aPos != aPos2 && aPos != aPos1)
    {
       if(aVal == '\"')
       {
        aString.erase(aPos,1);
       }
    }
  }

Answer 2

Scenario 1: Quotes after leading whitespace/before trailing whitespace

You may use a regex that will capture quotes at the start/end of string together with leading/trailing whitespace into Group 1, and will match quotes in all other contexts. Then you need to implement custom replacement for each of the possibilities: when Group 1 matches, you need to paste the whole match back, if not, replacr with ":

#include <iostream>
#include <cstdlib>
#include <string>
#include <regex>
using namespace std;
 
template<class BidirIt, class Traits, class CharT, class UnaryFunction>
std::basic_string<CharT> regex_replace(BidirIt first, BidirIt last,
    const std::basic_regex<CharT,Traits>& re, UnaryFunction f)
{
    std::basic_string<CharT> s;
 
    typename std::match_results<BidirIt>::difference_type
        positionOfLastMatch = 0;
    auto endOfLastMatch = first;
 
    auto callback = [&](const std::match_results<BidirIt>& match)
    {
        auto positionOfThisMatch = match.position(0);
        auto diff = positionOfThisMatch - positionOfLastMatch;
 
        auto startOfThisMatch = endOfLastMatch;
        std::advance(startOfThisMatch, diff);
 
        s.append(endOfLastMatch, startOfThisMatch);
        s.append(f(match));
 
        auto lengthOfMatch = match.length(0);
 
        positionOfLastMatch = positionOfThisMatch + lengthOfMatch;
 
        endOfLastMatch = startOfThisMatch;
        std::advance(endOfLastMatch, lengthOfMatch);
    };
 
    std::sregex_iterator begin(first, last, re), end;
    std::for_each(begin, end, callback);
 
    s.append(endOfLastMatch, last);
 
    return s;
}
 
template<class Traits, class CharT, class UnaryFunction>
std::string regex_replace(const std::string& s,
    const std::basic_regex<CharT,Traits>& re, UnaryFunction f)
{
    return regex_replace(s.cbegin(), s.cend(), re, f);
}
 
std::string my_callback(const std::smatch& m) {
  if (m.str(1).length() % 2 == 0) {
    return "&quot;";
  } else {
    return m.str(0);
  }
}
 
int main() {
    std::string str = "\"Hello people \" how are you doing \" what are \" you upto \"";
    cout << regex_replace(str, regex("(^\\s*\"|\"\\s*$)|\""), my_callback) << endl;
 
    return 0;
}

See the C++ demo. The callback implemenation by John Martin.

Scenario 2: Quotes at the start/end of string

You may use

std::regex("(?!^)\"(?!$)")

The (?!^) negative lookahead fails the match if the " is found at the start (^) and (?!$) fails if it is found at the end ($) of string.

See the regex demo

C++ demo:

#include <iostream>
#include <regex>
using namespace std;
 
int main() {
    std::string str = "\"Hello people \" how are you doing \" what are \" you upto \"";
    str = std::regex_replace(str, std::regex("(?!^)\"(?!$)"), "&quot;");
    std::cout << str << std::endl;
    return 0;
}

Output: "Hello people " how are you doing " what are " you upto "