expand file names that have environment variables in their path

Question

What\'s the best way to expand

${MyPath}/filename.txt to /home/user/filename.txt

or

%MyPath%/filename.txt to c:\\Document         


        

Answer 1

If you have the luxury of using C++11, then regular expressions are quite handy. I wrote a version for updating in place and a declarative version.

#include 
#include 

// Update the input string.
void autoExpandEnvironmentVariables( std::string & text ) {
    static std::regex env( "\\$\\{([^}]+)\\}" );
    std::smatch match;
    while ( std::regex_search( text, match, env ) ) {
        const char * s = getenv( match[1].str().c_str() );
        const std::string var( s == NULL ? "" : s );
        text.replace( match[0].first, match[0].second, var );
    }
}

// Leave input alone and return new string.
std::string expandEnvironmentVariables( const std::string & input ) {
    std::string text = input;
    autoExpandEnvironmentVariables( text );
    return text;
}

An advantage of this approach is that it can be adapted easily to cope with syntactic variations and deal with wide strings too. (Compiled and tested using Clang on OS X with the flag -std=c++0x)