For example, the word debacle would work because of debac , but seabed would

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Asked: June 6, 20262026-06-06T08:04:38+00:00 2026-06-06T08:04:38+00:00

For example, the word debacle would work because of debac , but seabed would

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For example, the word debacle would work because of debac, but seabed would not work because: 1. there is no c in any 5-character sequence that can be formed, and 2. the letter e appears twice. As another example, feedback would work because of edbac. And remember, the solution must be done using only regular expressions.

A strategy I attempted to implement was: match the first letter if it’s inside [a-e], and remember it. Then find the next letter in [a-e] but not the first letter. And so on. I wasn’t sure what the syntax was (or even if some syntax existed) so my code didn’t work:

open(DICT, "dictionary.txt");
@words = <DICT>;

foreach my $word(@words){

if ($word =~ /([a-e])([a-e^\1])([a-e^\1^\2])([a-e^\1^\2^\3])([a-e^\1^\2^\3^\4])/
){
    print $word;
}
}

I was also thinking of using (?=regex) and \G but I wasn’t sure how it would work out.

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Editorial Team · Answer 1 · 2026-06-06T08:04:39+00:00

/
   (?= .{0,4}a )
   (?= .{0,4}b )
   (?= .{0,4}c )
   (?= .{0,4}d )
   (?= .{0,4}e )
/xs

It’s probably results in faster matching to generate a pattern from all combinations.

use Algorithm::Loops qw( NextPermute );
my @pats;
my @chars = 'a'..'e';
do { push @pats, quotemeta join '', @chars; } while NextPermute(@chars);
my $re = join '|', @pats;

(This will get optimised into a trie in Perl 5.10+. Before 5.10, use Regexp::List.)

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For example, the word debacle would work because of debac , but seabed would

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