Regex expression to match whole word with special characters not working ? [duplicate]

Question

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• Regex expression to match whole word ? 1 answer

I was going through this question C#, Regex.Match whole words

It says for match whole word use "\bpattern\b" This works fine for match whole word without any special characters since it is meant for word characters only!

I need an expression to match words with special characters also. My code is as follows

class Program
{
    static void Main(string[] args)
    {
        string str = Regex.Escape("Hi temp% dkfsfdf hi");
        string pattern = Regex.Escape("temp%");
        var matches = Regex.Matches(str, "\\b" + pattern + "\\b" , RegexOptions.IgnoreCase);
        int count = matches.Count;
    }
}

But it fails because of %. Do we have any workaround for this? There can be other special characters like 'space','(',')', etc

Answer 1

If you have non-word characters then you cannot use \b. You can use the following

@"(?<=^|\s)" + pattern + @"(?=\s|$)"

Edit: As Tim mentioned in comments, your regex is failing precisely because \b fails to match the boundary between % and the white-space next to it because both of them are non-word characters. \b matches only the boundary between word character and a non-word character.

See more on word boundaries here.

Explanation

@"
(?<=        # Assert that the regex below can be matched, with the match ending at this position (positive lookbehind)
               # Match either the regular expression below (attempting the next alternative only if this one fails)
      ^           # Assert position at the beginning of the string
   |           # Or match regular expression number 2 below (the entire group fails if this one fails to match)
      \s          # Match a single character that is a “whitespace character” (spaces, tabs, and line breaks)
)
temp%       # Match the characters “temp%” literally
(?=         # Assert that the regex below can be matched, starting at this position (positive lookahead)
               # Match either the regular expression below (attempting the next alternative only if this one fails)
      \s          # Match a single character that is a “whitespace character” (spaces, tabs, and line breaks)
   |           # Or match regular expression number 2 below (the entire group fails if this one fails to match)
      $           # Assert position at the end of the string (or before the line break at the end of the string, if any)
)
"

Answer 2

If the pattern can contain characters that are special to Regex, run it through Regex.Escape first.

This you did, but do not escape the string that you search through - you don't need that.

Answer 3

output = Regex.Replace(output, "(?<!\w)-\w+", "")
output = Regex.Replace(output, " -"".*?""", "")