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19.1 Uses for Reentrant Scanners

However, there are other uses for a reentrant scanner. For example, you could scan two or more files simultaneously to implement a diff at the token level (i.e., instead of at the character level): 

    /* Example of maintaining more than one active scanner. */

    do {
        int tok1, tok2;

        tok1 = yylex( scanner_1 );
        tok2 = yylex( scanner_2 );

        if( tok1 != tok2 )
            printf("Files are different.");

   } while ( tok1 && tok2 );

Another use for a reentrant scanner is recursion. (Note that a recursive scanner can also be created using a non-reentrant scanner and buffer states. See Multiple Input Buffers.)

The following crude scanner supports the ‘eval’ command by invoking another instance of itself. 

    /* Example of recursive invocation. */

    %option reentrant

    %%
    "eval(".+")"  {
                      yyscan_t scanner;
                      YY_BUFFER_STATE buf;

                      yylex_init( &scanner );
                      yytext[yyleng-1] = ' ';

                      buf = yy_scan_string( yytext + 5, scanner );
                      yylex( scanner );

                      yy_delete_buffer(buf,scanner);
                      yylex_destroy( scanner );
                 }
    ...
    %%