const regex = new RegExp('(?<=_)[12]\\d{3}_[01]\\d_[0123]\\d(?=_)', 'gm')
const str = `CRC_recup_backup_2018_11_20_004003_1817970.bak
CRC_recup_backup_2018_11_21_004001_6027986.bak
CRC_recup_backup_2018_11_22_004001_7717997.bak
CRC_Test_backup_2018_11_16_004002_9068137.bak`;
const subst = `2020_08_09`;
// 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