XF 2.1 Regex Query

Hi all,

Newbie here:p

I have a custom field added onto the user registration.

I need to ensure they add only a Surname, followed by a comma, a space - then another name, and another - upto 100 characters all on one line

Example = Hallam, Wilson, Harris, Crowder, Lane, Crisp, Naylor, Downs


Any help required please:)