A function that checks a single string for consistency.
String must begin with 'x' and only consist of x's, a single period or none,
and may end with a percent symbol.
If string is consistent, TRUE is returned. Otherwise an error.
Usage
.check_fmt_string(x, variable, stat_name)
Value
a logical
Arguments
x
(string)
string to check
variable
(character) the variable whose statistic is to be formatted
stat_name
(character) the name of the statistic that is to be formatted