Question

For example, my situation:
I’m getting an input of “0”, “1”, “true” or “false”. (in any case)
what is preferred on terms of performance, code reading, any basically best-practice:

bool func(string param)
{
    string lowerCase = param;
    to_lower(lowerCase);
    if (lowerCase == "0" || lowerCase == "false")
    {
        return false;
    }
    if (lowerCase == "1" || lowerCase == "true")
    {
        return true;
    }
    throw ....
}

or:

bool func(string param)
{
    string lowerCase = param;
    to_lower(lowerCase);
    regex rxTrue  ("1|true");
    regex rxFalse ("0|false");

    if (regex_match(lowerCase, rxTrue)
    {
        return true;
    }
    if (regex_match(lowerCase, rxFalse)
    {
        return false;
    }
    throw ....
}

Answer 1

The second is somewhat clearer, and easier to extend (e.g.: accepting
"yes" and "no", or prefixes, with "1|t(?:rue)?)" and
"0|f(?:alse)?". With regards to performance, the second can (and
should) be made significantly faster by declaring the regex static
(and const, while you’re at it), e.g.:

static regex const rxTrue ( "1|true" , regex_constants::icase );
static regex const rxFalse( "0|false", regex_constants::icase );

Note too that by specifying case insensitivity, you’ll not have to
convert the input to lower case.