It's certainly more concise than my previous favorite, qtext = '[^\\x0d\\x22\\x5c\\x80-\\xff]' dtext = '[^\\x0d\\x5b-\\x5d\\x80-\\xff]' atom

It's certainly more concise than my previous favorite, qtext = '[^\\x0d\\x22\\x5c\\x80-\\xff]' dtext = '[^\\x0d\\x5b-\\x5d\\x80-\\xff]' atom = '[^\\x00-\\x20\\x22\\x28\\x29\\x2c\\x2e\\x3a-' + '\\x3c\\x3e\\x40\\x5b-\\x5d\\x7f-\\xff]+' quoted_pair = '\\x5c[\\x00-\\x7f]' domain_literal = "\\x5b(?:#{dtext}|#{quoted_pair})*\\x5d" quoted_string = "\\x22(?:#{qtext}|#{quoted_pair})*\\x22" domain_ref = atom sub_domain = "(?:#{domain_ref}|#{domain_literal})" word = "(?:#{atom}|#{quoted_string})" domain = "#{sub_domain}(?:\\x2e#{sub_domain})*" local_part = "#{word}(?:\\x2e#{word})*" addr_spec = "#{local_part}\\x40#{domain}" pattern = Regexp.new "\\A#{addr_spec}\\z", nil, 'n'