https://tools.ietf.org/html/rfc2616#section-3.6.1

   flex -8iCrfa <<eof
   xa \15|\12
   xb \15\12 
   %%
   ^[A-Fa-f0-9]+{xa}
   {xa}+[A-Fa-f0-9]+{xa}
   {xb}[A-Fa-f0-9]+{xb}
   %%
   int main(){ yylex();}
   int yywrap(){}
   eof
   exec cc -pipe -static lex.yy.c -o ctd