Case insensitive std::string.find()

Question

I am using std::string's find() method to test if a string is a substring of another. Now I need case insensitive version of the same thing. For string comparison I can always turn to stricmp() but there doesn't seem to be a stristr().

I have found various answers and most suggest using Boost which is not an option in my case. Additionally, I need to support std::wstring/wchar_t. Any ideas?

Answer 1

Why not just convert both strings to lowercase before you call find()?

tolower

Answer 2

You could use std::search with a custom predicate.

#include <locale>
#include <iostream>
#include <algorithm>
using namespace std;

// templated version of my_equal so it could work with both char and wchar_t
template<typename charT>
struct my_equal {
    my_equal( const std::locale& loc ) : loc_(loc) {}
    bool operator()(charT ch1, charT ch2) {
        return std::toupper(ch1, loc_) == std::toupper(ch2, loc_);
    }
private:
    const std::locale& loc_;
};

// find substring (case insensitive)
template<typename T>
int ci_find_substr( const T& str1, const T& str2, const std::locale& loc = std::locale() )
{
    T::const_iterator it = std::search( str1.begin(), str1.end(), 
        str2.begin(), str2.end(), my_equal<T::value_type>(loc) );
    if ( it != str1.end() ) return it - str1.begin();
    else return -1; // not found
}

int main(int arc, char *argv[]) 
{
    // string test
    std::string str1 = "FIRST HELLO";
    std::string str2 = "hello";
    int f1 = ci_find_substr( str1, str2 );

    // wstring test
    std::wstring wstr1 = L"ОПЯТЬ ПРИВЕТ";
    std::wstring wstr2 = L"привет";
    int f2 = ci_find_substr( wstr1, wstr2 );

    return 0;
}

Answer 3

Since you're doing substring searches (std::string) and not element (character) searches, there's unfortunately no existing solution I'm aware of that's immediately accessible in the standard library to do this.

Nevertheless, it's easy enough to do: simply convert both strings to upper case (or both to lower case - I chose upper in this example).

std::string upper_string(const std::string& str)
{
    string upper;
    transform(str.begin(), str.end(), std::back_inserter(upper), toupper);
    return upper;
}

std::string::size_type find_str_ci(const std::string& str, const std::string& substr)
{
    return upper(str).find(upper(substr) );
}

This is not a fast solution (bordering into pessimization territory) but it's the only one I know of off-hand. It's also not that hard to implement your own case-insensitive substring finder if you are worried about efficiency.

Additionally, I need to support std::wstring/wchar_t. Any ideas?

tolower/toupper in locale will work on wide-strings as well, so the solution above should be just as applicable (simple change std::string to std::wstring).

[Edit] An alternative, as pointed out, is to adapt your own case-insensitive string type from basic_string by specifying your own character traits. This works if you can accept all string searches, comparisons, etc. to be case-insensitive for a given string type.

Answer 4

There's a Gotw about this very subject :

http://www.gotw.ca/gotw/029.htm

Answer 5

If you want “real” comparison according to Unicode and locale rules, use ICU’s Collator class.