正则表达式，\b，指的是词边界处的空字符串，其中词是\w+，或[A-Za-z0-9_]+

The regular expression, \b, refers to the empty string at a word boundary, where word is \w+, or [A-Za-z0-9_]+

如果您每行有一个名称(名称周围没有其他空格)，您可以使用 ^{0}$ 和 re.M 按行搜索或 re.MULTILINE 标志

If you have one name per line (with no other whitespace around the names), you can search by line with ^{0}$ with the re.M or re.MULTILINE flag

看起来像这样:

def CheckUserExists(user):
    with open("C:/~/database.txt", 'r') as file:
        if re.search('^{0}$'.format(re.escape(user)), file.read(), flags=re.M):
            return True
        else:
            return False

username = input("Please enter you Username: ")
if CheckUserExists(username): # it's redundant to check if == True here
    print("You exist!")
else:
    print("This user does not exist...")

尽管评论和答案建议，如果你这样做

Although a comment and an answer suggest, if you do

if user in file.read()

你可能有误报.