Question

I am trying to parse the following line:

"\#" TEST #comment hello world

In my input, the #comment always comes at the end of the line. There may or may not be a comment, but if there is, its always in the end of the line.

I used the following Regex to parse it:

(\#.+)?

I have the RegexOption.RightToLeft on. I expected it to pull #comment hello world. But instead it is pulling "#" TEST #comment hello world"

Why is my Regex expression not pulling the right thing and what is the valid Regex expression I need to make it pull correctly?

Answer 1

I think you’ll find too many edge cases when trying to pull this off with regular expressions. Dealing with the quotes is what really complicates things, not to mention escape characters.

A procedural solution is not complicated, and will be faster and easier to modify as needs dictate. Note that I don’t know what the escape characters should be in your example, but you could certainly add that to the algorithm…

string CodeSnippet = Resource1.CodeSnippet;
StringBuilder CleanCodeSnippet = new StringBuilder();
bool InsideQuotes = false;
bool InsideComment = false;

Console.WriteLine("BEFORE");
Console.WriteLine(CodeSnippet);
Console.WriteLine("");

for (int i = 0; i < CodeSnippet.Length; i++)
{
    switch(CodeSnippet[i])
    {
        case '"' : 
            if (!InsideComment) InsideQuotes = !InsideQuotes;
            break;
        case '#' :
            if (!InsideQuotes) InsideComment = true;
            break;
        case '\n' :
            InsideComment = false;
            break;                       
    }

    if (!InsideComment)
    {
        CleanCodeSnippet.Append(CodeSnippet[i]);
    }
}

Console.WriteLine("AFTER");
Console.WriteLine(CleanCodeSnippet.ToString());
Console.WriteLine("");

This example strips the comments away from the CodeSnippet. I assumed that’s what you were after.

Here’s the output:

BEFORE
"\#" TEST #comment hello world
"ab" TEST #comment hello world
"ab" TEST #comment "hello world
"ab" + "ca" + TEST #comment
"\#" TEST
"ab" TEST

AFTER
"\#" TEST
"ab" TEST
"ab" TEST
"ab" + "ca" + TEST
"\#" TEST
"ab" TEST

As I said, you’ll probably need to add escape characters to the algorithm. But this is a good starting point.