Ruby: Testing a ruby string for a substring fails (substring is not recognized)

JavaScript's charCodeAt method tells me that the two characters are a different Unicode value. Ruby's .ord method tells me the same thing. You could check against those Unicode values more literally in Ruby, but I'd recommend finding a way to normalize the data instead of adding endless conditionals for unusual characters. It looks like that is a 0x0435 1077 CYRILLIC SMALL LETTER IE е according to a Unicode lookup table I found online.

Alternatively, here's one approach where you could just ban all Cyrillic characters. I used a full range of excluded characters so you could add exclusions as needed.

#!/usr/bin/env ruby

CYRILLIC_UNICODE_DECIMALS = *(1024..1273).freeze

for arg in ARGV
  # next unless arg.is_a?(String)

  arg.split('').each do |char|
    p char if CYRILLIC_UNICODE_DECIMALS.include?(char.ord)
  end
end

For reference, these are the .ord and .charCodeAt methods I used against your example. I started with JavaScript because it's a simple test in the browser console.

2.6.3 :005 > 'е'.ord
 => 1077
2.6.3 :006 > 'e'.ord
 => 101

'"е" == "e"'.charCodeAt(1)
1077
'"e" == "e"'.charCodeAt(1)
101