Regular expression to find a line containing certain characters and remove that line

Hi,

I have text file which has lot of character entries one line after another. I want to find all lines which start with :: and delete all those lines.

What is the regular expression to do this?

-AD

From stackoverflow

• Simple as:

  ^::
  

• ^::.*[\r\n]*
  

  If you're reading the file line-by-line you won't need the [\r\n]* part.

• Regular expressions don't "do" anything. They only match text.

  What you want is some tools that uses regular expressions to identify a line and then apply some command to those tools.

  One such tools is sed (there's also awk and many others). You'd use it like this:

  sed -e "/^::/d" < input.txt > output.txt
  

  The part "/^::/" tells sed to apply the following command to all lines that start with "::" and "d" simply means "delete that line".

  Or the simplest solution (which my brain didn't produce for some strange reason):

  grep -v "^::" input.txt > output.txt
  

  Dscoduc : I think you have forgotten the Regex.Replace function... That actually "does" something, doesn't it?

  Joachim Sauer : @Dcoduc: as you said: The function does something (its one of the tools I mentioned). The regular expression itself still only matches some text. It's the semantics of the function that defines what is to be done with the matched text.

  Dscoduc : Thanks for the clarification... I stand corrected...

• sed -i -e '/^::/d' yourfile.txt
  

  oylenshpeegul : I think this is perhaps the best answer, but it might be worth mentioning that not all versions of sed have a -i option.

• If you don't have sed or grep, find this and replace with empty string:

  ^::.*[\r\n]
  

• Thanks for the pointers:

  Following thing worked for me. After "::" any character was possiblly present in the text file so i gave:

  ^::[a-zA-Z0-9 I put all punctuation symbols here]*$

  -AD

  Manu : you don't need to match enything after the initial ^:: In your example you are forced to "account for" all the characters because you put a $ at the end.

  Alan Moore : If he's using a line-oriented tool like grep you're right. But he still hasn't said.

  Alan Moore : @goldenmean, what's preventing you from using .* instead of that monster character class?

  Dscoduc : I agree, it would be probably better to use a singleline option and add the .* to the expression.

  Alan Moore : Single-line? Why would you want the dot to match newline characters? If you read one line at a time, there won't be any newlines to match, and if you read the whole file into memory before processing, the dot-star will consume the rest of the file the first time it's applied.

• Here's my contribution in C#:

  Text stream:

  string stream = :: This is a comment line
  

  Syntax:

  Regex commentsExp = new Regex("^::.*", RegexOptions.Singleline);
  

  Usage:

  Console.WriteLine(commentsExp.Replace(stream, string.Empty));
  

  Alternatively, if I wanted to simply take a text file that included comments and produce an exact duplicate without the comment lines I could use a simple but effective combination of the type and findstr commandline tools:

  type commented.txt | findstr /v /R "^::" > uncommented.txt