The mac os x rename command doesn’t seem to support regular expressions. Based on osx - mac os x terminal batch rename - Stack Overflow, I used this command to rename files.
First, to see what it was going to do:
for f in _.*; do echo mv "$f" "${f/_/2019-06-05._}"; done
Then, removing the echo condom:
for f in _.*; do mv "$f" "${f/_/2019-06-05._}"; done
Well, apparently, I can’t use “real” regular expressions in that mv $f ${f//} syntax, or at least I couldn’t figure out how to do \n substitutions with that syntax. So, using sed, based on osx - Regex to batch rename files in OS X Terminal - Stack Overflow, this worked for a more complex renaming operation:
for f in 2017*; do sudo mv "$f" `echo $f | sed "s/^(....)(..)(..)/\1-\2-\3/g" -r`; done
Side note – the trouble using the mv $f ${f//} syntax was actually with a CentOS box. I haven’t tried more complex expressions on Mac OS X yet.