Repeated number regex

regex.js

// Regex to get any repeated numbers in a string
var repeatedNumber = /(\d)\d*\1/

/*
  (\d) -> first group (a number in this case) that will be checked by \1
  \d* -> any quantity of digits between the number to be checked and the "checker"
  \1 -> same stuff that is in the 1st group (in this case, a digit = \d)

e.g
  00:
    - (\d) = 0 (first group)
    - \d* = nothing (empty space)
    - \1 = checks that the second '0' matches the first group ('0')

  9239:
    - (\d) = 9 (first group)
    - \d* = 23
    - \1 = the second 9
    
  0434:
    - (\d) = 4 (first group)
    - \d* = 3
    - \1 = the second 4
*/