PWC131 - Find Pairs


On with TASK #2 from The Weekly Challenge #131. Enjoy!

The challenge

You are given a string of delimiter pairs and a string to search.

Write a script to return two strings, the first with any characters matching the “opening character” set, the second with any matching the “closing character” set.

Example 1:

    Delimiter pairs: ""[]()
    Search String: "I like (parens) and the Apple ][+" they said.


Example 2:

    Delimiter pairs: **//<>
    Search String: /* This is a comment (in some languages) */ <could be a tag>


The questions

At a first reading, I could not make too much of the challenge text. But I was probably tired, because after some time passed it makes perfectly sense.

The input string with the pairs is assumed to contain pairs of consecutive characters. Well… actually I didn’t bother to do anything about the expected encoding of the inputs, so it’s… whatever we get, most probably ASCII.

When the pair of consecutive characters actually contains the same character repeated twise, I’ll assume that it’s fine to find any occurrence independently of whether it’s an opening or a closing one. Whatever.

Last, there seems to be no constraint about escaping, so I’ll assume there is none.

The solution

This was pretty straightforward in Perl:

  • we divide the delimiters string putting the even-positioned character in a string, and the odd-positioned characters in another string (both go into an array `@delimiters);
  • then we do a non-destructive substitution using the strings of delimiters as character classes in the matching part.

Easier shown than described:

#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';

sub find_pairs ($delimiter_pairs, $search_string) {
   my @delimiters = ('', '');
   while ($delimiter_pairs) {
      $delimiters[$_] .= substr $delimiter_pairs, 0, 1, '' for 0, 1;
   return map { $search_string =~ s{[^\Q$_\E]+}{}rgmxs } @delimiters;
} ## end sub find_pairs

  for find_pairs(@ARGV
   ? @ARGV
   : ('""[]()', '"I like (parens) and the Apple ][+" they said.'));

My Raku-fu is not that strong, though.

First of all, I don’t particularly like the equivalent of the Perl’s substr form that allows putting another string instead of the one that is isolated. So I opted for splitting the input string using comb and working on the resulting array of character.

Second, it seems that it’s not possible to interpolate a variable into a character class (see this issue). Hence, using S/// was out of luck.

I eventually decided to split the input $search-string too, and use some smartmatching to get the filtering job done.

#!/usr/bin/env raku
use v6;

sub find-pairs ($delimiter-pairs, $search-string) {
   my @delimiters = [], [];
   my @chars = $delimiter-pairs.comb;
   (0, 1).map({@delimiters[$_].push: @chars.shift}) while @chars;
   return -> $d {
      $search-string.comb.grep({$^a ~~ $d.any}).join: '';

sub MAIN ($delimiter-pairs = '""[]()',
      $search-string = '"I like (parens) and the Apple ][+" they said.') {
   .put for find-pairs($delimiter-pairs, $search-string).List;

I’m not happy with this solution, the regular expression approach just seems much more natural.

Anyway, enough for this post… stay safe!

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