const regex = new RegExp('^.+ (\\w+) 3160 211 50 0(\\w+)\\d{3} .+$', 'gim')
const 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
`;
const subst = `$1 C$2`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', 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 JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions