#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11167676
use warnings;

local $_ = 'AB ABDA DCACCB AAA BSAA CAAB ACS ABA DBA BA DASSABACA A';

my $first = /(BA[ABC]{2})|(CA[CD]{2})|DA[SC]{2}/ ? $1 ? 1 : $2 ? 2 : 3
+ : 0;
print "first is $first\n";
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Outputs:

first is 2
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