Question

So having such code that filters letters, and / ".:= how to allow -?

#include <boost/regex.hpp>
#include <boost/algorithm/string/regex.hpp>

std::string filter_args(std::string args)
{
    std::cout << boost::erase_all_regex_copy(args, boost::regex("[^a-zA-Z0-9=\"/.: ]+")); 
    return boost::erase_all_regex_copy(args, boost::regex("[^a-zA-Z0-9=\"/.: ]+"));
}

Answer 1

Boost’s default Regex behaviour is Perl-compatible, and man perlre says:

You can specify a character class, by enclosing a list of characters
in [], which will match any character from the list. If the first
character after the “[” is “^”, the class matches any character not in
the list. Within a list, the “-” character specifies a range, so that
a-z represents all characters between “a” and “z”, inclusive. If you
want either “-” or “]” itself to be a member of a class, put it at the
start of the list (possibly after a “^”), or escape it with a
backslash. “-” is also taken literally when it is at the end of the
list, just before the closing “]”. (The following all specify the same
class of three characters: [-az], [az-], and [a\-z]. All are
different from [a-z], which specifies a class containing twenty-six
characters, even on EBCDIC-based character sets.) Also, if you try to
use the character classes \w, \W, \s, \S, \d, or \D as endpoints
of a range, the “-” is understood literally.

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So:

boost::erase_all_regex_copy(args, boost::regex("[^a-zA-Z0-9=\"/.: -]+"))

or

boost::erase_all_regex_copy(args, boost::regex("[^a-zA-Z0-9=\\-\"/.: ]+"))

(notice the double-backslash; one to escape for the string literal, and the second to escape for the regex).

I recommend the former.

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Always check out the documentation as your first port of call!