Is there a way to force bison and/or flex to restart scanning after I replace some token with something else?

My particular example would be with replacement for a specific word/string. If I want a word of hello to be replaced by echo hello, how can I get flex or bison to replace hello and then start parsing again (to pick up 2 words instead of just one). So it would be like:

• Get token WORD (which is a string type)
• If hello, replace token value with echo hello
• Restart parsing entire input (which is now echo hello)
• Get token WORD (echo)
• Get token WORD (hello)

I've seen very tempting functions like yyrestart(), but I don't really understand what that function in particular really accomplishes. Any help is greatly appreciated, thanks!