'  tom  harry  ' =~ m/(?<=tom).*(dick|harry|john)/ && print(1, $/);  # 1
'  harry  tom  ' =~ m/(?<=tom).*(dick|harry|john)/ && print(2, $/);  # (nothing)

##</code><code>##

'  tom  harry  ' =~ m/(?=.*tom).*(dick|harry|john)/ && print('A', $/);  # A
'  harry  tom  ' =~ m/(?=.*tom).*(dick|harry|john)/ && print('B', $/);  # B