Hi ! Thanks for the quick answer, this solved my problem but now I'm just having a little problem detecting string constants. I have the following regular expression in the lexer cte_string = "\".\""; and this grammar Graznido = tok.quack >> '(' >> (Expresion | tok.cte_string) >> *(',' >> (Expresion | tok.cte_string)) >> ')' >> ';'; it should accept something like this quack("hola", 1); but it fails to recognize it like something valid, it doesn't try to match "hola" with the cte_string, well that's what I think it's happening, because while debugging it tries to match it first with Expresion but fails and then it's like cte_string is not in the grammar. I dont't know what would be the better approach to detect a string constant with the parser. Thanks in advance I attach the code McJaml http://boost.2283326.n4.nabble.com/file/n3777837/Prueba.cpp Prueba.cpp -- View this message in context: http://boost.2283326.n4.nabble.com/problems-with-grammar-using-spirit-tp3775... Sent from the Boost - Dev mailing list archive at Nabble.com.