匹配到第二个括号的时候 值是 mo.group(2)

mo.group(1) 没匹配到就 None 了

def mark(mo):
    # print(mo.group(1))
    for i in range(1, 4):
        match_result = mo.group(i)
        if match_result is not None:
            print(i)
            return match_result


re_sub = re.sub(r'@(yangxg)|@(zengshao)|@(zmrenwu)', mark, '@yangxg @zengshao @zmrenwu')
print(re_sub)

这样写或许比较好些？输出符合你的要求

def mark(mo):
    print(mo.group(1))
    return mo.group(1)[1:]

data = re.sub(r'(@\w+)', mark, '@yangxg @zengshao @zmrenwu')
print data

import re
data = re.sub(r'@(\w+)', '\\1','@yangxg @zengshao @zmrenwu')
print data

result = re.sub(r'@(\w+)', lambda mo: mo.group(1), '@yangxg @zengshao @zmrenwu')