Whitelist characters

Hi people! I’m having a problem with a part of my script.

I want to check if the firstnamefield just have Allowed Letters.

	AllowedLetters = "qwertyuiopasdfghjklzxcvbnm" -- Allowed Letters
	local firstnamefield = vgui.Create("DTextEntry", mainFrame)
	firstnamefield:SetSize(200, 20)
	firstnamefield:SetPos(14, 137)

How can i check it?
If someone could help would be great. Naming fuctions to use below will help me to :wink: