Why you should not use (f)lex, yacc and bison
Lex and Yacc were the first popular and efficient lexers and parsers generators, flex and Bison were the first widespread open-source versions compatible with the original software. Each of these software has more than 30 years of history, which is an achievement in itself. For some people these are still the first software they think about when talking about parsing. So, why you should avoid them? Well, we found a few reasons based in our experience developing parsers for our clients.
Source: Why you should not use (f)lex, yacc and bison, an article by Federico Tomassetti.