What you want is something akin to a Raku (perl6) grammar, which was their enhancement to matching they developed to go beyond regular expressions.

    grammar IniFile {
        token key   { \w+ }
        token value { <-[\n;]>+ }
        token pair  { <key> \h* '=' \h* <value> \n+ }
        token TOP   { <pair>+ }
    }

    my $text = "user = alice\nrole = admin\n";
    my $match = IniFile.parse($text);

    if $match {
        say "Parsed successfully!";
    }

If you're not looking for that, and more specifically what you have shown above, there are often ways to accomplish that in certain languages. In Perl it's easy because you can use regex cars in a regex, so you can just assign `my $digit = qr/(? :0-9)/;` and then use it in the regex as you did (and Perl already supports non matching whitespace with a flag). For that matter, Perl regexes allow you to define chunks purely within the regex to be reused in a DEFINE block, so possibly that is a feature other regex engines have as well.