📘 Checking balanced parentheses using Raku

Check if the parentheses in a given string are balanced, i. e., whether every opening parenthesis has the corresponding closing one.

Let us limit the input strings with the strings containing parentheses () only and no other kinds of brackets {}, [], or <>. The text in between contains only letters and spaces. Empty parentheses are not allowed.

Prepare the test suite with different cases—a series of balanced examples and a series of strings with unbalanced parentheses.

my @tests = 'a',         '(a)',      '(a b c)', 'a (b)',
            '(b) a',     '(b (a))',  '( ( c))', 'a(b)c',

            'a (', 'a)', '(a) b c)', 'a b)',    '(b a',
            '((b (a))',  '((c)',    '(((a(((', ')a(';

For such a task, the best tool available in Perl 6 is grammars. Here is a simple grammar description that can recursively parse the above examples.

grammar Balanced {
    rule TOP {
        <expression>+
    }
    rule expression {
        | <:alpha>+ <expression>?        
        | '(' ~ ')' <expression>   
    }
}

The TOP rule says that the sentence is at least one expression. An expression is either a few letters (<:alpha>+) optionally followed by another expression or an expression in parentheses.

Notice the way parentheses are introduced in the expression rule:

'(' ~ ')' <expression>

This syntax allows keeping the opening and closing characters together and is synonymous with the following rule:

'(' <expression> ')'

Now we can parse the strings from the test suit with the Balanced grammar.

for @tests -> $test {    
    my $result = Balanced.parse($test);
    printf("%-12s is%s balanced\n", 
           $test,
           $result ?? '' !! ' not');
}

Depending on the success of parsing a string, the $result variable either contains a Match object or Nil. In the Boolean context, it is either True or False, and it defines what message is printed.

a            is balanced
(a)          is balanced
(a b c)      is balanced
a (b)        is balanced
(b) a        is balanced
(b (a))      is balanced
( ( c))      is balanced
a(b)c        is balanced
a)           is not balanced
(a) b c)     is not balanced
a b)         is not balanced
(b a         is not balanced
((b (a))     is not balanced
((c)         is not balanced
(((a(((      is not balanced
a (          is not balanced
)a(          is not balanced