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Showing content from https://en.cppreference.com/w/cpp/language/../regex/../string/basic_string/../byte/islower.html below:

std::islower - cppreference.com

Checks if the given character is classified as a lowercase character according to the current C locale. In the default "C" locale, std::islower returns a nonzero value only for the lowercase letters (abcdefghijklmnopqrstuvwxyz).

If islower returns a nonzero value, it is guaranteed that std::iscntrl, std::isdigit, std::ispunct, and std::isspace return zero for the same character in the same C locale.

The behavior is undefined if the value of ch is not representable as unsigned char and is not equal to EOF.

[edit] Parameters ch - character to classify [edit] Return value

Non-zero value if the character is a lowercase letter, zero otherwise.

[edit] Notes

Like all other functions from <cctype>, the behavior of std::islower is undefined if the argument's value is neither representable as unsigned char nor equal to EOF. To use these functions safely with plain chars (or signed chars), the argument should first be converted to unsigned char:

bool my_islower(char ch)
{
    return std::islower(static_cast<unsigned char>(ch));
}

Similarly, they should not be directly used with standard algorithms when the iterator's value type is char or signed char. Instead, convert the value to unsigned char first:

int count_lowers(const std::string& s)
{
    return std::count_if(s.begin(), s.end(),
                      // static_cast<int(*)(int)>(std::islower)         // wrong
                      // [](int c){ return std::islower(c); }           // wrong
                      // [](char c){ return std::islower(c); }          // wrong
                         [](unsigned char c){ return std::islower(c); } // correct
                        );
}
[edit] Example 
#include <cctype>
#include <clocale>
#include <iostream>
 
int main()
{
    unsigned char c = '\xe5'; // letter å in ISO-8859-1
 
    std::cout << "islower(\'\\xe5\', default C locale) returned "
              << std::boolalpha << (bool)std::islower(c) << '\n';
 
    std::setlocale(LC_ALL, "en_GB.iso88591");
    std::cout << "islower(\'\\xe5\', ISO-8859-1 locale) returned "
              << std::boolalpha << (bool)std::islower(c) << '\n';
 
}

Possible output:

islower('\xe5', default C locale) returned false
islower('\xe5', ISO-8859-1 locale) returned true
[edit] See also ASCII values characters

iscntrl
iswcntrl

isprint
iswprint

isspace
iswspace

isblank
iswblank

isgraph
iswgraph

ispunct
iswpunct

isalnum
iswalnum

isalpha
iswalpha

isupper
iswupper

islower
iswlower

isdigit
iswdigit

isxdigit
iswxdigit

decimal hexadecimal octal 0–8 \x0–\x8 \0–\10 control codes (NUL, etc.) ≠0 0 0 0 0 0 0 0 0 0 0 0 9 \x9 \11 tab (\t) ≠0 0 ≠0 ≠0 0 0 0 0 0 0 0 0 10–13 \xA–\xD \12–\15 whitespaces (\n, \v, \f, \r) ≠0 0 ≠0 0 0 0 0 0 0 0 0 0 14–31 \xE–\x1F \16–\37 control codes ≠0 0 0 0 0 0 0 0 0 0 0 0 32 \x20 \40 space 0 ≠0 ≠0 ≠0 0 0 0 0 0 0 0 0 33–47 \x21–\x2F \41–\57 !"#$%&'()*+,-./ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0 48–57 \x30–\x39 \60–\71 0123456789 0 ≠0 0 0 ≠0 0 ≠0 0 0 0 ≠0 ≠0 58–64 \x3A–\x40 \72–\100 :;<=>?@ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0 65–70 \x41–\x46 \101–\106 ABCDEF 0 ≠0 0 0 ≠0 0 ≠0 ≠0 ≠0 0 0 ≠0 71–90 \x47–\x5A \107–\132 GHIJKLMNOP
QRSTUVWXYZ 0 ≠0 0 0 ≠0 0 ≠0 ≠0 ≠0 0 0 0 91–96 \x5B–\x60 \133–\140 [\]^_` 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0 97–102 \x61–\x66 \141–\146 abcdef 0 ≠0 0 0 ≠0 0 ≠0 ≠0 0 ≠0 0 ≠0 103–122 \x67–\x7A \147–\172 ghijklmnop
qrstuvwxyz 0 ≠0 0 0 ≠0 0 ≠0 ≠0 0 ≠0 0 0 123–126 \x7B–\x7E \172–\176 {|}~ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0 127 \x7F \177 backspace character (DEL) ≠0 0 0 0 0 0 0 0 0 0 0 0

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