Javadoc
https://support.office.com/en-us/article/Define-and-use-names-in-formulas-4D0F13AC-53B7-422E-AFD2-ABD7FF379C64#bmsyntax_rules_for_names
Valid characters:
First character: { letter | underscore | backslash }
Remaining characters: { letter | number | period | underscore }
Cell shorthand: cannot be { "C" | "c" | "R" | "r" }
Cell references disallowed: cannot be a cell reference $A$1 or R1C1
Spaces are not valid (follows from valid characters above)
Name length: (XSSF-specific?) 255 characters maximum
Case sensitivity: all names are case-insensitive
Uniqueness: must be unique (for names with the same scope)