📘 Frequently used special variables in Raku

The $_ variable is the one similar to that in Perl 5, which is the default variable containing the current context argument in some cases. Like any other variable, the $_ is an object in Perl 6, even in the simplest use cases. For example, the recent example .say for @*ARGS implicitly contains the $_.say call. The same effect would give $_.say(), .say(), or just .say.

This variable is used as a default variable in other cases, for instance, during the match against regular expressions:

for @*ARGS {
    .say if /\d/;
}

This short code is equivalent to the following, which uses the smartmatch (~~) operator:

for @*ARGS {
    $_.say if $_ ~~ /\d/;
}

The result of matching against a regular expression is available in the $/ variable. To get the matched string, you may call the $/.Str method. So as to get the substrings, which were caught during the match, indices are used: $/[2] or, in a simpler form, $2.

"Perl’s Birthday: 18 December 1987" ~~ 
    / (\d+) \s (\D+) \s (\d+) /;
say $/.Str;
say $/[$_] for 0..2;

Here, we are looking for a date. In this case, the date is defined as a sequence of digits \d+, a space \s, the word having no digits \D+, another space \s, and some more digits \d+. If the match succeeded, the $/.Str slot contains the whole date, while the $/[0], $/[1], and $/[2] keep their parts (the small square corner brackets are part of the output to indicate the Match object, see Chapter 6):

18 December 1987
｢18｣
｢December｣
｢1987｣

Finally, the $! variable will contain an error message, for example, the one that occurred within a try block, or the one that happened while opening a file:

try {
    say 42/0;
}
say $! if $!;

If you remove the last line in this programme, nothing will be printed. This is because the try block masks any error output. Remove the try, and the error message reappears (the programme, itself, is terminated).