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^ (?: (?> # non backtracking for alternated tokens (?: \- # "-" operator (?=\d|\() # followed by digit or "(" ) | (?: (?<=\d|\)) # preceded by digit or ")" (?:\+|\/|\*) # basic operators except "-" (?=\d|\(|\-) # followed by digit or "(" or "-" ) | (?<parenthesis> # incremente balancing group "parenthesis" counter (?<=^|\+|\/|\-|\*|\() # preceded by start of line or basic operator or "(" \( # opening parenthesis "(" (?=\d|\(|\-) # followed by digit or "-" or "(" ) | (?<-parenthesis> # decrement balancing group "parenthesis" counter (?<=\d|\)) # preceded by digit or ")" \) # opening parenthesis ")" (?!\d) # not followed by digit ) | (?: (?<=\(|\-|\+|\*|\/|^) # preceded by start of line or basic operator or "(" (?:\d+(?:\,\d{1,4})?) # number with optional 4 decimal with comma as decimal marker (?=$|\)|\-|\+|\*|\/) # followed by end of line or ")" or basic operator ) ) + (?(parenthesis)(?!)) # test for balancing group "parenthesis" counter ) # unnamed group whole expression $