How to use ^ in regexp?

I would like start string with [a-z] and next symbols can be [a-zA-Z0-9], so I do:

^[a-z]*S

But this not work. Without "^" work good, but allow strings to start with capital letters and numbers.

Examples:

aaaa - should be ok
2aa - should be false
a - ok
1 - false
Asdf - false
aDS3r - ok
ssdfg2 - ok