l3regex is obsolete, replace with expl3

[tmalsburg]

Hi, I really like this package. Thank a lot for making it!

With TeX Live on current stable Ubuntu, I get a warning message saying that l3regex is obsolete:

*****************************************************************
** 
** Package l3regex is obsolete and has been removed!
** 
** Its functionality is now only provided as part of the expl3 package.
** 
** The old packages will be removed entirely at the end of 2018.
** 
** Therefore, please replace '\usepackage{l3regex}'
** with '\usepackage{expl3}' in your documents as soon as possible.
** 
*******************************************************************

Package l3regex Warning: This package is obsolete --- use 'expl3' instead on in
put line 42.