If the Regex field is empty, the location attribute field (with invalid regex) is not displayed when editing existing locations.
How to recreate this...
1.) Create a new location attribute, choose "Regex-Validated Text" and leave the field blank.
2.) Edit a pre-existing location; notice the new attribute field is not shown.
3.) Create a new location; notice the new attribute field is displayed.
This happens with OpenMRS Version: 1.9.7 Build 60bd9b, and on the web demo, Version: 1.9.3 Build f535e9.