#!/usr/bin/perl

use strict;
use warnings;

my $string = "A1 and A2 but not 1 and 2";

my $switch_hash = { 1 => 2, 2 => 1 };

my $regexp = '([A-Za-z])('.(join( '|', keys %$switch_hash )).')';

print "Regexp: $regexp\n";

my $comp_regexp = qr"$regexp";

$string =~ s/$comp_regexp/$1.$switch_hash->{$2}/ge;

print $string, "\n";