The following details the valid characters and restricted characters used for label names, and row/col/choice options when building your survey questions. This information applies to all question types.
1: Valid Characters
Use only a-z, A-Z, 0-9 and underscore character (_) for label names and question options.
Make sure that you do not use any of the following special characters, when changing your label names.
For example, in the graphic below you get "that label is invalid" error when using a period (.) between "q1" and "test."
2: Reserved Words for Labels
Decipher has a list of reserved words, that when used in a survey for a variable or label name will result in an error.
Here is the list of all Decipher Reserved Words.