I think the problem is the colon, not the exclamation point.
If you open Excel and try to manually edit a sheet name, the only characters it doesn’t let you type are [ ] * / \ ? :
If you paste one of those characters in, you get the following error: (Excel 2003)
While renaming a sheet or chart, you
entered an invalid name. Try one of
the following:
- Make sure the name you entered does not exceed 31 characters.
- Make sure the name does not contain any of the following
characters: : \ / ? * [ or ]- Make sure you did not leave the name blank.