Update: Enlil points out that this suffers from the same problem as the OP's own solution, highlighted by BrowserUk. 
#!/usr/bin/perl -wl
use strict;
$_="17341234";
s/(.)(?=...\1)|(?<=(.)...)\2/defined $1 ? "A" : "B"/eg;
print;
__END__
A7AAB2BB

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Makeshifts last the longest.

In reply to Re: Replace zero-width grouping? by Aristotle
in thread Replace zero-width grouping? by tinypig

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