I tried the following but without success 

```
[root@OBAMA~]# bash
[root@OBAMA~]# a=HelloWorld

[root@OBAMA~]# [[ $a == [A-Za-z] ]] && echo "YES ITS STRING"

```

( the command not prints anything )

```
[root@OBAMA~]# [[ $a == [A-Z][a-z] ]] && echo "YES ITS STRING"

```

( the command not prints anything )