Replacing elements in a QString based on a predicate

Question

I need to replace characters in a QString based on their QChar::category. In stdlib terms I want to

string.erase(std::remove_if(begin(string), end(string), 
                            [](QChar c) { 
                            QChar::Category cat = c.category(); 
                            return cat == .... || cat == ...; }), 
             string.end());

Alternatively, I'm happy with a regexp that works on unicode character categories that I can use for QString::replace.

Is that possible with QString or do I really need to turn the string in a std::vector<QChar> and back?

Edit: The categories I want to keep:

• for the first charater: $, _, or any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”
• for the rest: the first bullet plus any U+200C zero width non-joiner characters, U+200D zero width joiner characters, and characters in the Unicode categories “Non-spacing mark (Mn)”, “Spacing combining mark (Mc)”, “Decimal digit number (Nd)”, or “Connector punctuation (Pc)”.

I can do first/rest in multiple passes.

Answer 1

Qt provides its own ways to do such things. Whether it is good, or not is doubtful, but Qt idiomatic would be

QString result;
result.reserve(string.size());
foreach (const QChar& c, string) {
    if (is_good(c)) {
        result += c;
    }
}

Of course, you can do it with lambdas and std::for_each

std::for_each(string.begin(), string.end(),
                  [&result](QChar c)
                    {
                        if (is_good(c)) {result += c; }
                    }
    );

but it is not Qt idiomatic.

Note, that removing symbols from a string is slower, then adding new, if space was reserved, that is why the first code sample is fast.