如何在Python正则表达式中对字符进行分组后跟空格？

I have three strings:

我有三个字符串:

s1 = "A blah blah blah."             # match A
s2 = "Blah blah blah. A blah blah."  # match A
s3 = "Blah blah blah A."             # don't match 'A'
s1 = "A blah