Interesting. Do you have any examples? We use the regex python module to process the regular expressions in your changes file. regex tends not to have too many restrictions (unlike say re). Please do note that by the time your changes run, the USFM has been converted to USX and back again (probably) and so has been regularised somewhat. If you are having trouble with your regexes, take a look at the View&Edit tab for the final SFM of the file you are wanting to process and check that your regex matches that appropriately.