Code:
//var %string = this123.4,5is67Aa8string.9 | echo -ag $regsubex(%string,/[a-z]/g,)


Do note that this doesn't remove upercase letters, which you didn't explicitly asked for i know.

Code:
//var %string = this123.4,5is67Aa8string.9 | echo -ag $regsubex(%string,/[a-z]/gi,)


This will, in case you do need it to remove A-Z as well.

$maybe