JavaScript RegEx: Format string with dynamic length to block format

Question

The following RegEx formats given string to following output block pattern:

123 456 78 90 (= 3 digits 3 digits 2 digits 2 digits )

RegEx:

Answer 1

Using modern Javascript lookbehind that allows dynamic length assertion, you can use this regex to match:

/(?<=^(?:\d{6}(?:\d{2})*|(?:\d{3}){1,2}))(?=\d+$)/g

Just replace that with a single space i.e. " "

RegEx Demo

Lookbehind:

• (?<=: Start assertion
  • ^(?:\d{6}(?:\d{2})*: Make sure we have 6 digits followed by 0 or more digits pair behind
  • |: OR
  • (?:\d{3}){1,2}): Make we have one or two sets of 3 digit sets behind
• ): End assertion

Lookahead:

• (?=\d+$): Make sure we have at least a digit ahead