Shell Programming and Scripting

What is the string that would satisfy this expression: "^gs{3}$" ?

What is an expression that would satisfy all these strings: csafaq.exe, csafbc.exe, asdfcd.exe?

Also, To catch these files, :
gdnus1862[2].exe
gdnus10[1].exe
gdnus250[1].exe
gdnus2337[1].exe

I am using this regex. : ^gdnus[0,9]{1,5}.[0,9]{0,1}.\.exe$

But it is not working. What is wrong with it?

Your help is appreciated, thanks.

Code:

1. gsss
2. ^......\....$ or ^.s.f..\.exe$ or ^[ca]s[ad]f[abc][qcd]\.exe$
3  Use [0-9] instead of [0,9]

Awesome thank you...