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use strict;
my $a='ABCAAAABBBCCCCCAAABBBCCC';
my $b='ASRGRTGRT89579843rrrrrr';
print '$a matches',$/ if $a =~ /^[ABC]+$/;
print '$b matches',$/ if $b =~ /^[ABC]+$/;

perl -MYAPE::Regex::Explain -e 'print YAPE::Regex::Explain->new(qr/^[A
+BC]+$/)->explain;'
The regular expression:
...
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