请们帮小弟我写出上列的正则表达式

请大虾们帮我写出下列的正则表达式
(4) at least one lowercase letter, e.g., adg, akw
(5) any number of letters or digits, e.g., wery45t6, 25g73SH
(6) a letter (upper or lower case) followed by any number of letters or digits, e.g., R5yua7,f47FH

万分感谢~

------解决方案--------------------
[a-z]+
[a-z0-9A-Z]?
[a-zA-Z][a-z0-9A-Z]?
------解决方案--------------------
不好意思 结贴能给分的话给楼下 

Xiaos_hui