Project Euler：Problem 89 Roman numerals

For a number written in Roman numerals to be considered valid there are basic rules which must be followed. Even though the rules allow some numbers to be expressed in more than
one way there is always a "best" way of writing a particular number.
For example, it would appear that there are at least six ways of writing the number sixteen:

IIIIIIIIIIIIIIII

VIIIIIIIIIII

VVIIIIII

XIIIIII

VVVI

XVI
However, according to the rules only XIIIIII and XVI are valid,
and the last example is considered to be the most efficient, as it uses the least number of numerals.
The 11K text file, roman.txt (right
click and 'Save Link/Target As...'), contains one thousand numbers written in valid, but not necessarily minimal, Roman numerals; see About...
Roman Numerals for the definitive rules for this problem.
Find the number of characters saved by writing each of these in their minimal form.

Note: You can assume that all the Roman numerals in the file contain no more than four consecutive identical units.

化简成最简格式无非就是遇到DCCCC变成CM，LXXXX变成XC，VIIII变成IX，IIII变成IV，XXXX变成XL，CCCC变成CD

反正遇到这样的这些字符最后都变成2个字符

import re

ans=0
for line in open("roman.txt"):
    a=len(line)
    line=re.sub('DCCCC|LXXXX|VIIII|CCCC|XXXX|IIII','aa',line)
    b=len(line)
    ans=ans+a-b

print(ans)