Recently, several people have reported that the RegEx Find/Replace add-in for Excel has stopped working, displaying the following error message:

Compile error in hidden module: ThisWorkbook

The error appears to be a side effect of a recent update released by Microsoft. I don't know the details of exactly why this broke the add-in, and I'm not sure if it's caused by a Windows update or a Microsoft Office update. However, after scouring the Internet for a while, I came across a solution. Here are the steps to restore the add-in's functionality:
  1. Close Excel
  2. Open a command prompt with admin privileges
  3. Run the following two commands:
    • For 32-bit Windows:
      regsvr32 -u c:\windows\system32\mscomctl.ocx
      regsvr32 c:\windows\system32\mscomctl.ocx
    • For 64-bit Windows:
      regsvr32 -u c:\windows\syswow64\mscomctl.ocx
      regsvr32 c:\windows\syswow64\mscomctl.ocx
  4. Open Excel
Usually that solves the problem, although the same "Compile error in hidden module" error can sometimes be caused by corrupted Excel files. So if the above steps don't do the trick, there is one other possible solution you can try:
  1. Close Excel
  2. Open Windows Explorer
  3. In the Windows Explorer address bar, type %appdata%
    and press Enter
  4. In the resulting list of subfolders, find and open the Microsoft folder
  5. In the Microsoft folder, find and open the Forms subfolder
  6. In the Forms folder, rename or delete the following two files (if they are present):
    • comctllib.exd
    • mscomctllib.exd
  7. Open Excel (it will automatically recreate the aforementioned files as it needs them)
Hope that helps!