Example 11-9. examples/regex/assembly_source.txt

mv A, R3
mv R2, B
mv R1, R3
mv B, R4
add A, R1
add B, R1
add R1, R2
add R3, R3
mv X, R2

Example 11-10. examples/regex/assembly_process.pl

#!/usr/bin/perl
use strict;
use warnings;

# assuming there are no R4 values then 4 substitutions will do
s/R1/R4/g
s/R3/R1/g
s/R2/R3/g
s/R4/R2/g



# or without any assumption and in one substitution:
my %map = (
    R1  => 'R2',
    R2  => 'R3',
    R3  => 'R1',
);

s/(R[123])/$map{$1}/g