const regex = new RegExp('^(?<year>\\d{4})[-\\/](?<month>0[1-9]|1[0-2])[-\\/](?<day>0[1-9]|[1-2][0-9]|3[0-1])(?:[T\\s](?:(?<hour>0[0-9]|1[0-9]|2[0-4]):(?<minute>\\d{2}):(?<second>\\d{2})(?:\\.(?<nano>\\d+))?))?(?:(?<offset>[Zz]|(?:[+-])(?:[01]\\d|2[0-3])(?::?[0-5]\\d)?)?)$', 'gm')
const str = `2020-12-01T12:13:51.123+02:00
2020-12-01T12:13:51.123+0200
2020-12-01T12:13:51.123Z
2020-12-01T12:13:51.123
2020-12-01T12:13:51.123341512+02:00
2020-12-01T12:13:51.123341512+0200
2020-12-01T12:13:51.123341512Z
2020-12-01T12:13:51.123341512
2020-12-01T12:13:51+02:00
2020-12-01T12:13:51+0200
2020-12-01T12:13:51Z
2020-12-01T12:13:51
2020-12-01 12:13:51.123 +02:00
2020-12-01 12:13:51.123 +0200
2020-12-01 12:13:51.123Z
2020-12-01 12:13:51.123
2020-12-01 12:13:51.123341512 +02:00
2020-12-01 12:13:51.123341512 +0200
2020-12-01 12:13:51.123341512Z
2020-12-01 12:13:51.123341512
2020-12-01 12:13:51+02:00
2020-12-01 12:13:51+0200
2020-12-01 12:13:51Z
2020-12-01 12:13:51
2020-12-01+02:00
2020-12-01+0200
2020-12-01Z
2020-12-01
2020/12/01T12:13:51.123+02:00
2020/12/01T12:13:51.123+0200
2020/12/01T12:13:51.123Z
2020/12/01T12:13:51.123
2020/12/01T12:13:51.123341512+02:00
2020/12/01T12:13:51.123341512+0200
2020/12/01T12:13:51.123341512Z
2020/12/01T12:13:51.123341512
2020/12/01T12:13:51+02:00
2020/12/01T12:13:51+0200
2020/12/01T12:13:51Z
2020/12/01T12:13:51
2020/12/01+02:00
2020/12/01+0200
2020/12/01Z
2020/12/01
2039/15/23
2923/09/12
2023/23/51
2020-12-01T00:13:51
2020-12-01T01:13:51
2020-12-01T10:13:51
2020-12-01T19:13:51
2020-12-01T20:13:51
2020-12-01T24:13:51`;
const subst = `$1-$2-$3`;
// 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