regex_search and regex_replace with Boost

Question

I'm trying to loop among some strings delimited by a '$' pair in a line, replacing each match with a specific value in order to get an output line with all markers replaced but I'm stuck at the second match as I don't know how to concatenate the new replacement value:

const boost::regex expression( "\\$[\\w]+\\$" );
string fileLine( "Mr $SURNAME$ from $LOCATION$" );
string outLine;

string::const_iterator begin = fileLine.begin();
string::const_iterator end = fileLine.end();

boost::match_results<string::const_iterator> what;
boost::match_flag_type flags = boost::match_default;

while ( regex_search( begin, end, what, expression, flags ) ) {
  actualValue = valuesMap[what[0]];

  ostringstream t( ios::out | ios::binary );
  ostream_iterator<char, char> oi( t );

  boost::regex_replace( oi, begin, end, expression, actualValue, 
                        boost::match_default | boost::format_first_only );
  outLine.append( t.str() );
  begin = what[0].second;
}

The problem is in the outLine.append( t.str() ) as the concatenation is not done properly because after the first match, the outLine holds already some of the characters preceding the next match.

Answer 1

Since you request only first value in a string to be replaced (by using boost::format_first_only flag) original string

"Mr $SURNAME$ from $LOCATION$"

will be converted into

"Mr ACTUAL_VAL from $LOCATION$"

on first iteration and then

" from ACTUAL_VAL"

will be appended to it since you explicitly set begin to "what[0].second. so final output is

"Mr ACTUAL_VAL from $LOCATION$ from ACTUAL_VAL"

which is not what you need. Here is working example that has side effect - it modifies fileLine:

   const boost::regex expression( "\\$[\\w]+\\$" );
    string fileLine( "Mr $SURNAME$ from $LOCATION$" );
    string outLine;

    string::const_iterator begin = fileLine.begin();
    string::const_iterator end = fileLine.end();

    boost::match_results<string::const_iterator> what;
    boost::match_flag_type flags = boost::match_default;

    while ( regex_search( begin, end, what, expression, flags ) ) 
    {
        const char* actualValue = valuesMap[what[0]];

        ostringstream t( ios::out | ios::binary );
        ostream_iterator<char, char> oi( t );

        boost::regex_replace( oi, begin, end, expression, 
`enter code here`actualValue, boost::match_default | boost::format_first_only );

        fileLine.assign(t.str());
        begin = fileLine.begin();
        end = fileLine.end();        
    }

    std::cout << fileLine << std::endl;

If you don't want to modify fileLine, then you should make "begin" and "end" to mark the beginning and end of sliding window that contains exactly one pattern.

Answer 2

Though I'm not 100% sure about your intent, I presume your goal is replacing each matched substring in fileLine with the corresponding value of valuesMap .
If so, the following code might meet your purpose:

  ...same as your code...

  while ( regex_search( begin, end, what, expression, flags ) ) {
    outLine.insert( outLine.end(), begin, what[0].first );
    outLine += valuesMap[what[0]];
    begin = what[0].second;
  }

  outLine.insert( outLine.end(), begin, end );

Hope this helps