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

A nicely idiomatic non-regex solution is to use String#count:

"MT3G22AH".count("^AGHTM0-9").zero?  # => true
"TAR34".count("^AGHTM0-9").zero?     # => false

The inverse also works, if you find it more readable:

"MT3G22AH".count('AGHTM0-9') == "MT3G22AH".size  # => true

Take your pick.

For longer strings, both methods here perform significantly better than regex-based options.