C++ Replacing non-alpha/apostrophe with spaces in a string - string

Question

I am reading in a text file and parsing the words into a map to count numbers of occurrences of each word on each line. I am required to ignore all non-alphabetic chars (punctuation, digits, white space, etc) except for apostrophes. I can figure out how to delete all of these characters using the following code, but that causes incorrect words, like "one-two" comes out as "onetwo", which should be two words, "one" and "two".
Instead, I am trying to now replace all of these values with spaces instead of simply deleting, but can't figure out how to do this. I figured the replace-if algorithm would be a good algorithm to use, but can't figure out the proper syntax to accomplish this. C++11 is fine. Any suggestions?
Sample output would be the following:
"first second" = "first" and "second"
"one-two" = "one" and "two"
"last.First" = "last" and "first"
"you're" = "you're"
"great! A" = "great" and "A"
// What I initially used to delete non-alpha and white space (apostrophe's not working currently, though)
// Read file one line at a time
while (getline(text, line)){
istringstream iss(line);
// Parse line on white space, storing values into tokens map
while (iss >> word){
word.erase(remove_if(word.begin(), word.end(), my_predicate), word.end());
++tokens[word][linenum];
}
++linenum;
}
bool my_predicate(char c){
return c == '\'' || !isalpha(c); // This line's not working properly for apostrophe's yet
}

Answer 1

bool my_predicate(char c){
return c == '\'' || !isalpha(c);
}
Here you're writing that you want to remove the char if it is and apostrophe or if it is not an alphabetical character.
Since you want to replace these, you should use std::replace_if() :
std::replace_if(std::begin(word), std::end(word), my_predicate, ' ');
And you should correct your predicate too :
return !isalpha(c) && c != '\'';

Answer 2

You could use std::replace_if to pre-process the input line before sending it to the istringstream. This will also simplify your inner loop.
while (getline(text, line)){
replace_if(line.begin(), line.end(), my_predicate, ' ');
istringstream iss(line);
// Parse line on white space, storing values into tokens map
while (iss >> word){
++tokens[word][linenum];
}
++linenum;
}