Validate that string contains only allowed characters in Ruby

Question

How can I test whether a Ruby string contains only a specific set of characters?

For example, if my set of allowed characters is \"AGHTM\" plus digits <

Answer 1

allowed = "AGHTM"
allowed = /\A[\d#{allowed}]+\z/i

"MT3G22AH" =~ allowed #⇒ truthy
"TAR34" =~ allowed #⇒ falsey