Retaining just needed characters in string in C++ - c++

Question

I have a string of the form:
http://stackoverflow.com/q""uestions/ask/%33854#/á
Now I want to delete all characters from this string except alphnumeric and ://.So that the output string becomes:
http://stackoverflow.com/questions/ask/33854/á
I know I can traverse this string character by character and remove unnecessary characters. But is there some function in some standard library which may help me remove unwanted characters. If i know the unwanted characters then I can use std::remove and std::replace to selectively remove or replace. But here I do not know the unknown characters, I only know the characters which I want to retain.
Is there some way by which I may retain only the necessary characters and remove the unwanted characters.
gcc version which I am using is:
gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4)
EDIT: I also want to include characters like á. I dont know what they are called. I know they are not alph-numeric. But I am not getting how to check for them

Answer 1

Since your compiler is ancient and regex support is relatively recent in gcc (from gcc 4.9 forward), regexes are not an option. We'll use the erase-remove idiom, with a named function because Gcc 4.4 does not yet support lambdas.
#include <algorithm>
#include <iostream>
#include <locale>
#include <string>
// true for characters that should be removed
bool is_special_character(char c) {
std::locale loc("your_locale_string_here");
return !std::isalnum(c, loc) && c != ':' && c != '/' && c != '.';
}
int main()
{
std::string s = "http://stackoverflow.com/q\"\"uestions/ask/%33854#";
// interesting part here
s.erase(std::remove_if(s.begin(), s.end(), is_special_character), s.end());
std::cout << s << '\n';
}

Answer 2

You will want to use std::remove_if and define a predicate to return false only if the characters are the ones you want to retain.
You'll also want to resize the string to the new length after you do this process. As an example:
#include <string>
#include <algorithm>
#include <iostream>
#include <locale>
bool is_special_char(char c)
{
return !( std::isalnum(c) || c == ':' || c == '/' || c == '.');
}
int main()
{
std::string s = "http://stackoverflow.com/q\"\"uestions/ask/\%33854#";
std::cout << s << std::endl;
std::string::iterator new_end = std::remove_if(s.begin(), s.end(), is_special_char);
s.resize(new_end - s.begin());
std::cout << s << std::endl;
}
will output
http://stackoverflow.com/q""uestions/ask/%33854#
http://stackoverflow.com/questions/ask/33854
If you want to incorporate unicode characters you need to use a wstring instead of a string, an example using this (and incorporating Wintermute's nice use of the erase/remove idiom) would be.
#include <string>
#include <algorithm>
#include <iostream>
#include <locale>
bool is_special_char(wchar_t c)
{
return !( std::iswalnum(c) || c == ':' || c == '/' || c == '.');
}
int main()
{
std::locale::global( std::locale("en_US.UTF-8") ); //Set the global locale to Unicode
std::wstring s = L"http://stáckoverflow.com/q\"\"uestions/ask/%33854#";
std::wcout << s << std::endl;
s.erase( std::remove_if(s.begin(), s.end(), is_special_char), s.end() );
std::wcout << s << std::endl;
}
which will output
http://stáckoverflow.com/q""uestions/ask/%33854#
http://stáckoverflow.com/questions/ask/33854

Answer 3

But here I do not know the unknown characters, I only know the characters which I want to retain.
Whitelist the characters you want to retain using a char array for example. Then run through each character in your string and remove it if it isn't in the whitelist.

Answer 4

You could try something like that :
std::string str ("This is an example sentence.");
std::cout << str << '\n';
// "This is an example sentence."
str.erase (10,8); // ^^^^^^^^
std::cout << str << '\n';
// "This is an sentence."
str.erase (str.begin()+9); // ^
std::cout << str << '\n';
// "This is a sentence."
str.erase (str.begin()+5, str.end()-9); // ^^^^^
std::cout << str << '\n';
// "This sentence."