They don't do 'regular expressions' at steemit, as far as I know. Or at least, in the code that checks for a valid username, in steemd, is implemented as a very long, almost page full of switch statements. I have pointed this out a few times but I don't expect it got fixed. Apparently also the devs didn't know about such things as 'edge cases' as the announcemennt of v0.19.1rc1 item 4 or so indicates. A virtual transaction on zero interest. Really quality coders we got here.
Time to stop pointing at it and showing how it's supposed to be done, I think.