linux正则问题，加单引号、双引号和不加引号有什么区别？

[root@vm01 tmp]# cat test_rule.txt
Mr.Li Ming said:
he was the honest man in LampBrother.
123despise him.

But since Mr.Shen Chao came,
he never saaaid those words.

because,actuaaaally,
Mr.Shen Chao is the most honest man!

Later,Mr.Li ming soid his hot body.

sd
sad
saad

[root@vm01 ~]# grep a* /tmp/test_rule.txt
abc:aa
/tmp/test_rule.txt:he never saaaid those words.
/tmp/test_rule.txt:because,actuaaaally,
/tmp/test_rule.txt:saad
[root@vm01 ~]# grep 'a*' /tmp/test_rule.txt
Mr.Li Ming said:
he was the honest man in LampBrother.
123despise him.

But since Mr.Shen Chao came,
he never saaaid those words.

because,actuaaaally,
Mr.Shen Chao is the most honest man!

Later,Mr.Li ming soid his hot body.

sd
sad
saad