Ruby: How to find out if a character is a letter or a digit?

The simplest way would be to use a Regular Expression:

def numeric?(lookAhead)
  lookAhead =~ /[0-9]/
end

def letter?(lookAhead)
  lookAhead =~ /[A-Za-z]/
end

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.

Regular expression is an overkill here, it's much more expensive in terms of performance. If you just need a check is character a digit or not there is a simpler way:

def is_digit?(s)
  code = s.ord
  # 48 is ASCII code of 0
  # 57 is ASCII code of 9
  48 <= code && code <= 57
end

is_digit?("2")
=> true

is_digit?("0")
=> true

is_digit?("9")
=> true

is_digit?("/")
=> false

is_digit?("d")
=> false