how to replace all occurences of a string or character, unless already escaped?

Question

Is there a nice and elegant way (using boost::algorithm::replace maybe?) to replace all occurrences of a character in a string - unless preceded by a backslash?

so that

std::string s1("hello 'world'");
my_replace(s1, "'", "''"); // s1 becomes "hello ''world''"

std::string s2("hello \\'world'"); // note: only a single backslash in the string
my_replace(s2, "'", "''"); // s2 becomes "hello \\'world''"

Using boost::regex, this can be done using:

std::string my_replace (std::string s, std::string search, std::string format) {
  boost::regex e("([^\\\\])" + search);
  return boost::regex_replace(s, e, "\\1" + format);
}

But I prefer not to use boost::regex due to performance reasons. boost::algorithm::replace looks like a good fit, but I can't figure out exactly how.

Answer 1

Here is a simple algorithm that does the job:

#include <iostream>
#include <string>

using namespace std;

string replace(char c, string replacement, string s)
{
    string chars = string("\\") + c;

    size_t pos = s.find_first_of(chars);
    while (pos != string::npos)
    {
        char& ch = s[pos];    
        if (ch == '\\')
        {
            pos = s.find_first_of(chars, pos + 2);
        }
        else if (ch == c)
        {
            s.replace(pos, 1, replacement);
            pos = s.find_first_of(chars, pos + replacement.length());
        }
    }

    return s;
}

int main()
{
    cout << replace('\'', "''", "hello \\'world'");
}

UPDATE:

Following @BenVoigt's suggestion, I reformulated the algorithm to avoid in-place operations. That should bring further performance improvement:

string replace(char c, string replacement, string const& s)
{
    string result;
    size_t searchStartPos = 0;

    string chars = string("\\") + c;
    size_t pos = s.find_first_of(chars);
    while (pos != string::npos)
    {
        result += s.substr(searchStartPos, pos - searchStartPos);
        if (s[pos] == '\\')
        {
            result += string("\\") + c;
            searchStartPos = pos + 2;
        }
        else if (s[pos] == c)
        {
            result += replacement;
            searchStartPos = pos + 1;
        }

        pos = s.find_first_of(chars, searchStartPos);
    }

    return result;
}