#JavaScript (ES6), 25 bytes

s=>/(.+?)\1*$/.exec(s)[1]

Try it online!

###Regular expression

   +-------> 1st (and only) capturing group: 1 or more character(s), non greedily
   |   +---> reference to the capturing group, repeated 0 to N times
  _|_  | +-> end of string
 /   \/ \|
/(.+?)\1*$/

JavaScript (ES6), 25 bytes

s=>/(.+?)\1*$/.exec(s)[1]

Try it online!

Regular expression

   +-------> 1st (and only) capturing group: 1 or more character(s), non greedily
   |   +---> reference to the capturing group, repeated 0 to N times
  _|_  | +-> end of string
 /   \/ \|
/(.+?)\1*$/

#JavaScript (ES6), 25 bytes

s=>/(.+?)\1*$/.exec(s)[1]

Try it online!

#JavaScript (ES6), 25 bytes

s=>/(.+?)\1*$/.exec(s)[1]

Try it online!

###Regular expression

   +-------> 1st (and only) capturing group: 1 or more character(s), non greedily
   |   +---> reference to the capturing group, repeated 0 to N times
  _|_  | +-> end of string
 /   \/ \|
/(.+?)\1*$/