# Perl Weekly Challenge 194: regular expressions everywhere!

This post presents my solutions to the Perl Weekly Challenge 194.
I keep doing the Perl Weekly Challenge in order to mantain my coding skills in good shape, as well as in order to learn new things, with particular regard to Raku, a language that I love.
This week, I solved the following tasks:
The PL/Perl implementations are very similar to a pure Perl implementation, even if the PostgreSQL environment could involve some more constraints. Similarly, the PL/PgSQL implementations help me keeping my PostgreSQL programming skills in good shape.

## PWC 194 - Task 1 - Raku Implementation

The first task was about receiving an input from a digital clock, so in the form `hh:mm` where one digit was missing and substituted with a `?`. The task required to find out the max value such missing digit can assume. The problem is quite simple, the only fact is that the missing digit can change its value depending on the position. I decided to implement it by means of a few regular expressions able to catch where the digit is missing and which value it can assume:

``````sub MAIN( Str \$what ) {

given ( \$what ) {
when ( / ^ \? \d ':' \d ** 2 \$ / )     { 2.say and exit  }
when ( / ^ <[01]> \? ':' \d ** 2 \$ / ) { 9.say and exit  }
when ( / ^ 2 \? ':' \d ** 2 \$ / )      { 3.say and exit  }
when ( / ^ \d ** 2 ':' \? \d \$ / )     { 5.say and exit  }
when ( / ^\d ** 2 ':' \d \? \$ / )      { 9.say and exit  }
}
}

``````

The PL/Perl implementation is a little smarter than that.

## PWC 194 - Task 2 - Raku Implementation

The second task was about an input string of repeated characters. The task was about to find out if, removing a single character, all the remaining characters will have the same number of occurencies. It is not clear to me if the same frequency is required or not, so I assume only the same number of occurencies over the whole string.

``````sub MAIN( Str \$what where { \$what ~~ / ^ <[a..z]>+ \$ / } ) {

my \$counter = Bag.new: \$what.comb;

"1".say and exit if ( \$counter.values.max - \$counter.values.min == 1
&& \$counter.keys.grep( { \$counter{ \$_ } == \$counter.values.max } ) == 1 );

"0".say;

}

``````

The idea is quite simple: I count the occurencies of every character, keeping them into the `\$counter` Bag. The program allows for only one character being different in size than the others, so there must be only one character that has a max value and such value must be greater than `1` than the other characters.

## PWC 194 - Task 1 - PL/Perl Implementation

A smarter approach than the Raku solution: I capture all the places where a digit can appear, and then elaborate what to do next:

``````CREATE OR REPLACE FUNCTION
RETURNS int
AS \$CODE\$
my (\$what) = @_;

if ( \$what =~ / ^ ([\d?]) ([\d?]) : ([\d?]) ([\d?]) \$ /x ) {
if ( \$1 eq '?' ) {
return 9;
}
elsif ( \$2 eq '?' ) {
return 3 if \$1 == 2;
return 9;
}
elsif ( \$3 eq '?' ) {
return 5;
}
else {
return 9;
}
}

return undef;

\$CODE\$
LANGUAGE plperl;

``````

## PWC 194 - Task 2 - PL/Perl Implementation

Similar to the Raku implementation, with an hand-made Bag approach:

``````CREATE OR REPLACE FUNCTION
RETURNS int
AS \$CODE\$
my ( \$what ) = @_;

my %counter;
my ( \$max, \$min ) = ( 0, 0 );

for ( split '', \$what ) {
\$counter{ \$_ }++;
\$min = \$counter{ \$_ } if ( ! \$min || \$min > \$counter{ \$_ } );
\$max = \$counter{ \$_ } if ( ! \$max || \$max < \$counter{ \$_ } );
}

return 0 if ( \$max - \$min != 1 );
return 0 if ( grep( { \$counter{ \$_ } == \$max }  keys %counter ) != 1 );
return 1;

\$CODE\$
LANGUAGE plperl;

``````

## PWC 194 - Task 1 - PL/PgSQL Implementation

An implementation following the PL/Perl approach: I match with a regular expression, then loop over the single letter/digit extracted. I need an index in order to know where I am within the string, as well as a previous character to keep in order to see what was the digit before the current one.

``````CREATE OR REPLACE FUNCTION
RETURNS int
AS \$CODE\$
DECLARE
needle char;
idx    int := 0;
prev   char;
BEGIN
FOREACH needle IN ARRAY regexp_match( what, '^([\d?])([\d?]):([\d?])([\d?])\$' ) LOOP
IF needle <> '?' THEN
idx := idx + 1;
prev := needle;
CONTINUE;
END IF;

IF idx = 0 THEN
RETURN 2;
ELSEIF idx = 1 THEN
IF prev = '2' THEN
RETURN 3;
ELSE
RETURN 9;
END IF;
ELSEIF idx = 2 THEN
RETURN 5;
ELSE
RETURN 9;
END IF;
END LOOP;
RETURN NULL;
END
\$CODE\$
LANGUAGE plpgsql;

``````

## PWC 194 - Task 2 - PL/PgSQL Implementation

I use a temporary table to count as a bag. I do insert every letter into the table, and in case of conflict, I issue an upsert to update the table and the letter counter. Then Iām able to process the `current_max` and `current_min`, as well as the number of letters having a `curent_max` counter.

``````CREATE OR REPLACE FUNCTION
RETURNS int
AS \$CODE\$
DECLARE
t text;
current_max int;
current_min int;
current_count int;
BEGIN
CREATE TEMPORARY TABLE IF NOT EXISTS counter ( l char, c int, PRIMARY KEY(l) );
TRUNCATE counter;

FOR t IN SELECT v FROM regexp_split_to_table( what, '' ) v LOOP
INSERT INTO counter AS cnt ( l, c )
VALUES ( t, 1 )
ON CONFLICT (l)
DO UPDATE SET c = cnt.c + 1;
END LOOP;

SELECT max(c), min(c)
INTO current_max, current_min
FROM counter;

IF current_max - current_min <> 1 THEN
RETURN 0;
END IF;

SELECT count(*)
INTO current_count
FROM counter
WHERE c = current_max;

IF current_count <> 1 THEN
RETURN 0;
END IF;

RETURN 1;
END
\$CODE\$
LANGUAGE plpgsql;

``````

The conditions evaluated are the same as in the PL/Perl implementation.

The article Perl Weekly Challenge 194: regular expressions everywhere! has been posted by Luca Ferrari on December 5, 2022