#Use a regular expression that matches letters & digits:
def letter?(lookAhead)
  lookAhead.match?(/[[:alpha:]]/)
end

def numeric?(lookAhead)
  lookAhead.match?(/[[:digit:]]/)
end

#These are called POSIX bracket expressions, and the advantage of them is
#that unicode characters under the given category will match. For example:

'ñ'.match?(/[A-Za-z]/)     #=> false
'ñ'.match?(/\w/)           #=> false
'ñ'.match?(/[[:alpha:]]/)  #=> true

#You can read more in Ruby’s docs for regular expressions.
#https://ruby-doc.org/core/Regexp.html