A regex to find if a letter occurs exactly $n times.
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#!/usr/bin/perl
# http://perlmonks.org/?node_id=1201500
use strict;
use warnings;
my @words = glob '{i,x}' x 6;
print "@words\n";
my $n = 1; # number of times a letter occurs in a word
my @oneletter = grep
/^(?|
(?=.*?(.)(?!.*\1))
(?:
(?:(?!\1).)*
\1
){$n}
(?:(?!\1).)*
|
(?=.*(.))
(?:
(?:(?!\1).)*
\1
){$n}
(?:(?!\1).)*
)$
/x, @words;
print "\n@oneletter\n\n", scalar @oneletter, "\n";
Update: fails for some test cases
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