2.1.1108 Part 4 Section 3.18.9, ST_CellSpan (Cell Span Type)

a.   The standard states that the ST_CellSpan simple type is referenced by the ST_CellSpan simple type.

Office defines this simple type to be referenced by the ST_CellSpans simple type.

b.   The standard allows any string to be a valid CellSpan.

In Office, the numerical values separated by the colon specify the first and last columns of the span using one-based column indices. The values shall be positive and never exceed the maximum number of columns within the spreadsheet document. The value to the left of the colon shall be less than or equal to the value to the right of the colon. 

This restriction matches the following regular expression pattern: \d+:\d+.