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examples/regex/assembly_source.txt

mv A, R3
mv R2, B
mv R1, R3
mv B1, R4
add A, R1
add B, R1
add R1, R2
add R3, R3
add R21, X
add R12, Y
mv X, R2

examples/regex/assembly_process.py

import sys
import re

if len(sys.argv) != 2:
    exit(f"Usage: {sys.argv[0]} FILENAME")

filename = sys.argv[1]

with open(filename) as fh:
    code = fh.read()

code = re.sub(r'R1', 'R4', code)
code = re.sub(r'R3', 'R1', code)
code = re.sub(r'R2', 'R3', code)
code = re.sub(r'R4', 'R2', code)

print(code)