Course of Raku / Addendum 🆕 / Types and text machines / Regexes and grammars / Exercises / Validate identifiers

Solution: Validate identifiers

Here is a possible solution to the task.

Code

for <count total2 2fast my-var _hidden> -> $name {
    my $ok = $name ~~ / ^ <[A..Za..z_]> <[A..Za..z0..9_]>* $ /;

    say "$name: { $ok ?? 'valid' !! 'invalid' }";
}

🦋 You can find the source code in the file validate-identifier.raku.

Output

count: valid
total2: valid
2fast: invalid
my-var: invalid
_hidden: valid

Comments

1. The anchors ^ and $ force the pattern to cover the whole string, so a single stray character like the hyphen in my-var makes it invalid.

2. The first character class allows a letter or underscore; the second, repeated with *, additionally allows digits — matching the classic identifier rule exactly.

3. The second class can also be written as \w, the built-in shorthand for a word character (a letter, a digit, or an underscore):

   my $ok = $name ~~ / ^ <[A..Za..z_]> \w* $ /;

   One difference to be aware of: \w in Raku is Unicode-aware, so a name such as café also passes — which happens to match Raku itself, where my $café = 1; is perfectly legal. The spelled-out class <[A..Za..z0..9_]> keeps the check strictly ASCII.

4. There is a twist, though: in Raku itself my-var is a valid identifier! Raku allows a hyphen (or an apostrophe, as in isn't) inside a name, as long as it is followed by a letter — which is why subroutines like is-prime read so naturally. To validate Raku identifiers, allow such groups after the classic part:

   for <count total2 2fast my-var _hidden> -> $name {
       my $ok = $name ~~ / ^ <[A..Za..z_]> \w* [ '-' <[A..Za..z]> \w* ]* $ /;
   
       say "$name: { $ok ?? 'valid' !! 'invalid' }";
   }

   Each bracketed group [ '-' <[A..Za..z]> \w* ] accepts a hyphen only when a letter follows, so my-var is now reported as valid, while 2fast — and strings like a- or a-1 — still are not.

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