>>> origin = "hello alex bcd alex lge alex acd 19"
>>> r=re.sub("a\w+","999",origin,2)
>>> r
'hello 999 bcd 999 lge alex acd 19'

“a\w+”：表示匹配以a开头的单词

"999"：表示替换成的值

origin：原始字符串

2：只匹配2个

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