Halp with regex

Hey /prog/,
Lets say I have a file with many lines,
each line consists of a very very long random number (string)
Is it possible to write a regular expression to find the longest series of repeated numbers shared by all lines
aka
120123456789948761238746102347106416239847113958719385
217490812741234567890182472103571938547019112413956138756183756183756817356817356
192346112345678978932671658917612746183724681734682173649187245

(I bolded it for readability)

tldr;
how does I finded biggest repeaded series of numbers contained in all lines? (using regex or something one-line'ish)

The most straight-forward way would be to find the shortest line and then to check every possible substring of it to see if it occurs in every other line.
It's not possible to do in a single regex.