Broken keyboard

You are given a string containing only English letters and a list of broken keys - letters that cannot be typed. Write a script to return the total words in the given sentence that can be typed completely.

Examples

Example 1
Input: $str = 'Hello World', @keys = ('d')
Output: 1
With broken key 'd', we can only type the word 'Hello'.

Example 2
Input: $str = 'apple banana cherry', @keys = ('a', 'e')
Output: 0

Example 3
Input: $str = 'Coding is fun', @keys = ()
Output: 3
No keys broken.

Example 4
Input: $str = 'The Weekly Challenge', @keys = ('a','b')
Output: 2

Example 5
Input: $str = 'Perl and Python', @keys = ('p')
Output: 1

Analysis

My algorithm is:

• Concatenate the broken keys as $keys (eg 'abc')
• Find any words matching \w*[$keys]\w* and delete them
• Count the remaining words

The regex deletes an occurrence of zero or more letters followed by a broken key followed by zero or more letters. The g modifier repeats that until there are no more matches. It doesn't matter if the word contains more than one broken key: as soon as the first one is detected, the whole word is deleted.

See also week 275 task 1

This is a practical, well-documented technical blog post that demonstrates a regex-focused approach to problem-solving with strong emphasis on robustness and error handling. Peter provides clean, working solutions with comprehensive examples and thoughtful edge case consideration.

Try it 

#!/usr/bin/perl

# Blog: http://ccgi.campbellsmiths.force9.co.uk/challenge

use v5.26;    # The Weekly Challenge - 2025-09-29
use utf8;     # Week 341 - task 1 - Broken keyboard
use warnings; # Peter Campbell Smith

binmode STDOUT, ':utf8';
use Encode;

broken_keyboard('Hello world', ['d']);
broken_keyboard('apple banana cherry', ['a', 'e']);
broken_keyboard('Coding is fun', []);
broken_keyboard('Perl and Python', ['p']);
broken_keyboard('aaaaa ccc b', ['a', 'b']);

sub broken_keyboard {
    
    my ($string, $keys, $count);
    
    # initialise and concatenate broken keys
    no warnings 'uninitialized';
    $string = $_[0];
    $keys .= $_ for @{$_[1]};
    
    # delete words containing broken keys
    $string =~ s|\w*[$keys]\w*||gi if $keys;
    
    # count the remaining words
    $count ++ while $string =~ m|\w+|gi;
    
    # report
    say qq[\nInput:  \$string = '$_[0]', \@keys = (']  . join(q[', '], @{$_[1]}) . q[')];
    say qq[Output: ] . ($count + 0);
}

9 lines of code

Output from script

Input:  $string = 'Hello world', @keys = ('d')
Output: 1

Input:  $string = 'apple banana cherry', @keys = ('a',
   'e')
Output: 0

Input:  $string = 'Coding is fun', @keys = ('')
Output: 3

Input:  $string = 'Perl and Python', @keys = ('p')
Output: 1

Input:  $string = 'aaaaa ccc b', @keys = ('a', 'b')
Output: 1