In OCL, the String split function gets expects a char - but OCL defaults single characters to String. To solve this, do this:

SomeString.split('X'.toCharArray)

This OCL SomeString.split('X') will give you the error: 31:System.String does not conform to Collection(System.Char)

An example of creating a Collection(Part1:System.String+Part2:System.String) which is a tuple that you can iterate to - for example, do search and replace:

'A, B;
X, Y'.Split(';'.toCharArray)->collect(pair | pair.Split(','.toCharArray)->at(1), pair.Split(','.toCharArray)->at(2))

RGB := '241, 55, 45'; '#' + RGB.split(','.toCharArray)->
  collect(s|Integer.parse(s))->
    collect(i|i.toString('x'))->asSeparatedList('')

If you want to do the reverse, look at asSeparatedList.