Python argparse regex

The type keyword argument can take any callable that accepts a single string argument and returns the converted value. If the callable raises argparse.ArgumentTypeError, TypeError, or ValueError, the exception is caught and a nicely formatted error message is displayed.

import argparse
import re 
from uuid import uuid4

def my_regex_type(arg_value, pat=re.compile(r"^[a-f0-9A-F]{32}$")):
    if not pat.match(arg_value):
        raise argparse.ArgumentTypeError("invalid value")
    return arg_value

parser = argparse.ArgumentParser()
parser.add_argument('hex', type=my_regex_type)

args = parser.parse_args([uuid4().hex])