How do I know if a string has a letter in it?

I'm trying to make a retro-style nuking system on which you put coordinates onto where the bomb will go.

I want to make it so that when they input a letter into the textbox It shows a ERROR, I'm still learning string manipulation, if I can get a link to a place that actually explains how to do it (I tried checking for 'string' in Roblox studio forums but It just says what it does and not how to use it in my case.

Hopefully no one has asked this before and I can get proper answers.

Good Day!

Sorry guys but I found the answer! Case you yourself want to know just do this!

if string.match(string, '%a') then
--do something
end

Thanks for trying!