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    <p>I know next to no VHDL, so let's say you want to replace all single line comments in a (Java) source file with multi-line comments:</p> <pre><code>//foo </code></pre> <p>should become:</p> <pre><code>/* foo */ </code></pre> <p>You need to let the lexer match single line comments, of course. But you should also make sure it recognizes multi-line comments because you don't want <code>//bar</code> to be recognized as a single line comment in:</p> <pre><code>/* //bar */ </code></pre> <p>The same goes for string literals:</p> <pre><code>String s = "no // comment"; </code></pre> <p>Finally, you should create some sort of catch-all rule in the lexer that will match any character.</p> <p>A quick demo:</p> <pre><code>grammar T; parse : (t=. {System.out.print($t.text);})* EOF ; Str : '"' ('\\' . | ~('\\' | '"'))* '"' ; MLComment : '/*' .* '*/' ; SLComment : '//' ~('\r' | '\n')* { setText("/* " + getText().substring(2) + " */"); } ; Any : . // fall through rule, matches any character ; </code></pre> <p>If you now parse input like this:</p> <pre><code>//comment 1 class Foo { //comment 2 /* * not // a comment */ String s = "not // a // comment"; //comment 3 } </code></pre> <p>the following will be printed to your console:</p> <pre><code>/* comment 1 */ class Foo { /* comment 2 */ /* * not // a comment */ String s = "not // a // comment"; /* comment 3 */ } </code></pre> <p>Note that this is just a quick demo: a string literal in Java could contain Unicode escapes, which my demo doesn't support, and my demo also does not handle char-literals (the char literal <code>char c = '"';</code> would break it). All of these things are quite easy to fix, of course.</p>
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    1. COYou're welcome @Andy.
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