#! perl -slw
use strict;

my $re = qr[
	(.)
	(?=(.)(.))
	(?= .* 
		(?: 
			\1\2\3 
			| 
			\1\3\2 
			| 
			\2\1\3 
			| 
			\2\3\1 
			| 
			\3\1\2 
			| 
			\3\2\1 
		)
	.* $)
]x;

while( <DATA> ) {
	chomp;
	print "'$_'", $_ !~ m[$re]g ? ' passed' : 'failed';
}

__DATA__
abcdbca
abcba
abcdabe

##</code><code>##

P:\test>test3
'abcdbca'failed
'abcba'failed
'abcdabe' passed

##</code><code>##

my $re = qr[
	(.) (?=(.)(.))
	(?= .* 
		(?: 
			\1 (?: \2\3 | \3\2 ) 
			| 
			\2 (?: \1\3 | \3\1 )
			| 
			\3 (?: \1\2 | \2\1 )
		)
	.* $)
]x;