There are two important regular expressions used within Tplyr. The
format_string expression is the expression to parse format strings. This is
what is used to make sense out of strings like 'xx (XX.x%)' or 'a+1 (A.a+2)'
by inferring what the user is specifying about number formatting.
The 'format_group' regex is the opposite of this, and when given a string of
numbers, such as ' 5 (34%) [9]' will return the separate segments of numbers
broken into their format groups, which in this example would be ' 5',
'(34%)', and '[9]'.