I have a verilog gate-level netlist generated by Synopsys Design Compiler.

DC was directed to flatten all busses and ports when creating the netlist, and the module declaration is given below:

module DES_SBOX1 ( CLK, \ptext[5] , \ptext[4] , \ptext[3] , \ptext[2] , 
    \ptext[1] , \ptext[0] , \key[5] , \key[4] , \key[3] , \key[2] , 
    \key[1] , \key[0] , \ctext[3] , \ctext[2] , \ctext[1] , 
    \ctext[0]  );

My problem is that when I go to use this module in a testbench, I receive syntax errors because I am not sure how to reference the ports names (because of the back-slash.)

Here is an attempt at instantiating one of these modules:

DES_SBOX1 feistel( .CLK(CLK_tb),

...and here is the error message from Modelsim PE:

** Error: C:\Users\Kristin\Desktop\des_feistel_90nm\testbench.v(14): (vlog-2730) Undefined variable: 'ptext'.

** Error: C:\Users\Kristin\Desktop\des_feistel_90nm\testbench.v(14): near "[": syntax error, unexpected '[', expecting ')'

I've also tried using the back-slash in the instantiation itself, like so:


..but this also gives a syntax error.

I'm wondering what the significance of the back-slash is in the first place?

Any ideas of how to correctly instantiate this module are greatly appreciated.

Thanks in advance!



2 Answers 2


It seems like you are running into Verilog's Escape identifiers gotcha:

§2.7.1 Escaped identifiers

Escaped identifiers shall start with the backslash character () and end with white space (space, tab, newline). They provide a means of including any of the printable ASCII characters in an identifier (the decimal values 33 through 126, or 21 through 7E in hexadecimal).

Neither the leading backslash character nor the terminating white space is considered to be part of the identifier. Therefore, an escaped identifier \cpu3 is treated the same as a nonescaped identifier cpu3.

So not only you need to properly write down escaped port name, but you also need to make sure you don't forget to put a whitespace at the end. For example:

DES_SBOX1 feistel( .CLK(CLK_tb),
                 .\ptext[5] (ptext_tb[5]),
                 .\ptext[4] (ptext_tb[4]),

Hope it helps. Good Luck!

  • \$\begingroup\$ Great explanation! Wow, something so subtle.. thanks a lot for pointing it out. Is that quote from the IEEE standard reference? \$\endgroup\$
    – kbarber
    Sep 28, 2012 at 17:23
  • \$\begingroup\$ @kbarber: yes it is :) \$\endgroup\$
    – user8459
    Sep 28, 2012 at 18:10

Wow, I've never seen a netlist with port names escaped like that. I supposed it's because they have [] as part of the port name.

Try dropping the port names from the port connection list. That is, use the positional mapping instead of connecting by name. Normally this would be discouraged because "by name" is more readable, but it may get you past the compile error in this case.


DES_SBOX1 feistel(CLK_tb,        // CLK
                 ptext_tb[5],    // ptext[5]
                 ptext_tb[4],    // ptext[4]
                 ptext_tb[3],    // ptext[3]
                 ptext_tb[2],    // ptext[2]
                 ptext_tb[1],    // ptext[1]
                 ptext_tb[0],    // ptext[0]
                 1'b0,           // key[5]
                 1'b0,           // key[4]
                 1'b0,           // key[3]
                 1'b0,           // key[2]
                 1'b0,           // key[1]
                 1'b0,           // key[0]
                 ctext_tb[5],    // ctext[5]
                 ctext_tb[4],    // ctext[4]
                 ctext_tb[3],    // ctext[3]
                 ctext_tb[2],    // ctext[2]
                 ctext_tb[1],    // ctext[1]
                 ctext_tb[0]     // ctext[0]

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