formula_peg.txt in rubyfromexcel-0.0.13

- old
+ new
@@ -2,14 +2,14 @@
 expression                       = string_join | comparison | arithmetic | thing
 thing                            = function | brackets | any_reference | string | percentage | number | boolean | prefix | named_reference 
 argument                         = expression | null
 function                        := /[A-Z]+/ `'(' space argument? (space `',' space argument)* space `')'
 brackets                        := `'(' space expression+ space `')'
-string_join                     := thing (space `"&" space thing)+
+string_join                     := (arithmetic | thing) (space `"&" space (arithmetic | thing))+
 arithmetic                      := thing (space operator space thing)+
 comparison                      := (arithmetic | thing) space comparator space (arithmetic | thing)
 comparator                      := '>=' | '<=' | '<>' | '>' | '<' | '='
-string                          := `'"' /[^"]*/ `'"'
+string                          := `'"' /(""|[^"])*/ `'"'
 any_reference                    = external_reference | any_internal_reference
 any_internal_reference           = table_reference | local_table_reference | quoted_sheet_reference | sheet_reference | sheetless_reference
 percentage                      := /[-+]?[0-9]+\.?[0-9]*/ `'%'
 number                          := /[-+]?[0-9]+\.?[0-9]*([eE][-+]?[0-9]+)?/
 operator                        := '+' | '-' | '/' | '*' | '^'