parsing "\ENABLED" - Unrecognized escape sequence \E

Question

string line = "ena";    

// pattern == "\\ENABLED"; pattern is input by user (see comments below)
string pattern = Console.ReadLine();

foreach (Match match in Regex.Matches(line, pattern,RegexOptions.CultureInvariant))
{
    Console.WriteLine("match");
}

In above example Regex.Matches() class giving exception parsing "\ENABLED" -

Unrecognized escape sequence \E.

`"\\\\ENABLED";` you have to escape *twice* for C# and for Regex. Or you can put `@"\\ENABLED"` — Dmitry Bychenko, Mar 26 '19 at 09:59
pattern i.e "\\ENABLED" is input by user in the form of textbox so how to handle in textbox — ND's, Mar 26 '19 at 10:05
if `pattern` is input by *user*, it can well by syntactically incorrect and we should be ready this: we have to add *exception catching* and let user know that the `pattern` is incorrect. — Dmitry Bychenko, Mar 26 '19 at 10:07

Dmitry Bychenko · Answer 1 · 2019-03-26T12:04:33.607

The immediate cause of the exception is that regular expressions as well as C# treat backslash (\) as a special symbol, e.g. in the

  \bABC\p{L}

pattern both \ have a special meaning. So you have to escape \ twice (for C# and for regex):

  const string pattern = "\\\\ENABLED";

or (in order to make pattern be more readable) you can turn string into verbatim one:

  const string pattern = @"\\ENABLED";

If it's user (see comments to the question) who provides the pattern we should be ready to get a syntactically incorrect one; we can try catching exception(s):

  Regex regex = null; 

  while (true) { 
    Console.WriteLine("Please, provide the pattern: ");

    try {
      // Try to get pattern from user
      // Trim() - let's be nice and remove leading / trailing spaces
      // TimeSpan.FromSeconds(10) - user can provide a very bad pattern,
      // we don't want to freeze forever (10 seconds at most)
      regex = new Regex(
        Console.ReadLine().Trim(),
        RegexOptions.CultureInvariant,
        TimeSpan.FromSeconds(10));

      // if pattern is syntactically valid
      break;
    }
    catch (ArgumentException e) {
      Console.WriteLine($"Pattern is invalid: {e.Message}");
    }  
  }

  // We have a valid Regex regex based on user pattern; let's use it:

  try {
    foreach (Match match in regex.Matches(line)) {
      Console.WriteLine("match");
    } 
  }
  catch (RegexMatchTimeoutException) {
    //TODO: pattern is bad one, let user know it
  }

Same idea if pattern is provided in a TextBox:

  Regex regex = null; 

  try {
    regex = new Regex(
      patternTextBox.Text.Trim(), 
      RegexOptions.CultureInvariant,
      TimeSpan.FromSeconds(10));
  }
  catch (ArgumentException e) {
    if (patternTextBox.CanFocus)
      patternTextBox.Focus();

    MessageBox.Show($"Incorrect pattern: {e.Message}");

    return;
  } 

  try {
    foreach (Match match in regex.Matches(line)) {
      ...
    }
  }
  catch (RegexMatchTimeoutException) {
    //TODO: pattern is bad one, let user know it
  }

parsing "\ENABLED" - Unrecognized escape sequence \E

1 Answers1