Regular Expressions in Verilog

// Verilog doesn't have built-in support for regular expressions.
// However, we can demonstrate pattern matching using casex statements,
// which allow for "don't care" bits in the comparison.

module regex_example;
  
  // Define some test strings as 8-bit ASCII
  reg [63:0] test_string1 = "peach";
  reg [95:0] test_string2 = "peach punch";
  
  // Define a pattern to match "p__ch" where _ can be any character
  wire [39:0] pattern = "p??ch";
  
  // Function to check if a string matches the pattern
  function automatic bit match_pattern;
    input [63:0] str;
    begin
      casex (str)
        {pattern, 24'b?}: match_pattern = 1'b1;
        default: match_pattern = 1'b0;
      endcase
    end
  endfunction
  
  // Main simulation block
  initial begin
    // Test whether the pattern matches a string
    $display("Match result: %b", match_pattern(test_string1));
    
    // Find the first occurrence of the pattern in a longer string
    reg [7:0] i;
    reg found;
    found = 0;
    for (i = 0; i < 12; i = i + 1) begin
      if (match_pattern(test_string2[95:32-i*8])) begin
        $display("Pattern found at index: %d", i);
        found = 1;
        break;
      end
    end
    if (!found) $display("Pattern not found");
    
    // Demonstrate "replacement" by changing matching parts
    reg [95:0] modified_string;
    modified_string = test_string2;
    for (i = 0; i < 12; i = i + 1) begin
      if (match_pattern(modified_string[95:32-i*8])) begin
        modified_string[95:32-i*8] = "<fruit>";
      end
    end
    $display("Modified string: %s", modified_string);
  end

endmodule

This Verilog code demonstrates concepts similar to regular expressions using pattern matching capabilities available in hardware description languages:

We define a simple pattern p??ch where ? represents any character, similar to . in regex.
The match_pattern function uses a casex statement to check if a given string matches our pattern. This is analogous to the MatchString function in the Go example.
We then use this function to perform operations similar to those in the Go example:
- Checking if a string matches the pattern
- Finding the first occurrence of the pattern in a longer string
- Modifying parts of a string that match the pattern
Note that Verilog doesn’t have built-in string manipulation functions, so we work with fixed-width bit vectors representing ASCII strings.
The initial block is used to run our tests, similar to the main function in the Go example.
We use $display for output, which is Verilog’s equivalent of fmt.Println.

This example demonstrates how pattern matching concepts can be applied in a hardware description language, even though it doesn’t have native regex support like Go does.

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Regular Expressions in Verilog