Raku Books / Using Raku / Regexes and Grammars / Substitutions with regexes

83. %Templating% engine

Implement a simple templating engine, which substitutes values in placeholders of the form %name%.

The objective is to create a function that takes a template string and a hash with named values and does the substitution. So, let us prepare and pass them to a function. Notice that, in Raku, it is possible to pass a hash as easy as you pass a scalar (see also Task 65, Passing arrays to subroutines).

my $template = 'Hello, %name%! Welcome to %city%!';
my %data = (
    name => 'Klara',
    city => 'Karlovy Vary',
);

say process_template($template, %data);

Inside the function, the hash is passed as a single hash variable.

sub process_template($template is copy, %data) {
    $template ~~ s:g/ '%' (\w+) '%' /%data{$0}/;
    return $template;
}

The function modifies the value of the first argument; this is why it is marked with the is copy trait.

The regex is global, which is turned on by the :g regex adverb. A regex is looking for words between the two percentage symbols. In Raku, non-alphanumeric characters must be quoted or escaped, so both '%' and \% are accepted. The second part of the replacement is using the matched value as the key for fetching the value from the %data hash.

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