Here's one solution. 

local $_ = "A3xyz45M98AB7Z9"; 
s/(?<=\d)\B(?=\D)/:/g;
print $_, $/;

[download]

Update: I see this is the same as the solution of Roy Johnson.

In reply to Re: Best way to put : between fields in a string by ambrus
in thread Best way to put : between fields in a string by eyepopslikeamosquito

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