Suppose I have a list of characters [a,b,c] and I want to write a regular expression such that
any string is accepted if it has all the elements in the character list at-least once and the characters can appear in any order in the string.

Example of accepted strings

> 
> `abc, aabbbc, bbaac, cab`
> 
> 
> 

Example of strings not accepteed

> 
> `aaabb, bab, caa, aacd, deeff`
> 
> 
>