countUntilLastMatchOf

import std.conv : to;

{
    enum haystack = "haystack";
    alias pred = (n) => (n == 'a');
    enum index = haystack.countUntilLastMatchOf!pred;
    static assert((index == 5), index.to!string);
}
{
    enum haystack = "haystack";
    alias pred = (n) => (n < 'a');
    enum index = haystack.countUntilLastMatchOf!pred;
    static assert((index == -1), index.to!string);
}
{
    enum haystack = "haystack";
    alias pred = (n) => (n == char.init);
    enum index = haystack.countUntilLastMatchOf!pred;
    static assert((index == -1), index.to!string);
}
{
    enum haystack = string.init;
    alias pred = (n) => (n == '\0');
    enum index = haystack.countUntilLastMatchOf!pred;
    static assert((index == -1), index.to!string);
}
{
    import std.uni : toLower;
    enum haystack = "HAYSTACK";
    alias pred = (n) => (n.toLower == 'a');
    enum index = haystack.countUntilLastMatchOf!pred;
    static assert((index == 5), index.to!string);
}
{
    enum haystack = "haystack";
    enum pred = "a == 'y'";
    enum index = haystack.countUntilLastMatchOf!pred;
    static assert((index == 2), index.to!string);
}