re = /^.+ (\w+) 3160 211 50 0(\w+)\d{3} .+$/im
str = 'RD32 MK12001 3160 211 50 02100123001 SERVER LOCAL - STREET
RD32 MK13103 3160 211 50 02100124001 SERVER LOCAL - CITY
RD32 J4834-00009-92051 MRDOOP 3160 211 50 021005237001 SERVER GIVEN NAME
RD32 B5509-00000-00522 JPPK 3160 211 50 02123133001 SERVER GIVEN NAME
'
subst = '$1 C$2'
result = str.gsub(re, subst)
# Print the result of the substitution
puts result
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html