Question

Does the System.Text.RegularExpressions namespace offer me anything to discover whether an input string has (“abc[0-9]” or “^aeiou$”) or has not (“abc123”) metacharacters? Or do I have to check manually for non-escaped characters in a certain list?

Answer 1

You have at least three options:

1. Use Regex.Escape and compare the result:

   private static bool ContainsMetaCharacters(string s)
   {
       if (String.IsNullOrEmpty(s))
       {
           return false;
       }
       string escaped = Regex.Escape(s);
       return !escaped.Equals(s, StringComparison.Ordinal);
   }
   

2. Apply Regex.Escape to each character to see if the escaped value is different:

   private static bool ContainsMetaCharacters(string s)
   {
       if (String.IsNullOrEmpty(s))
       {
           return false;
       }
       for (int i = 0; i < s.Length; i++)
       {
           if (Regex.Escape(s.Substring(i,1))[0] != s[i])
           {
               return true;
           }
       }
       return false;
   }
   

3. Create your own based on the fact that the metachars shouldn’t change:

   private static readonly char[] _MetaChars = 
       new char[] 
       { '\t', '\n', '\f', '\r', 
         ' ', '#', '$', '(', ')', 
         '*', '+', '.', '?', 
         '[', '\\', '^', '{', '|' };
   
   private static bool ContainsMetaCharacters(string s)
   {
       if(String.IsNullOrEmpty(s))
       {
           return false;
       }
       return s.IndexOfAny(_MetaChars) >= 0;
   }
   

The third approach offers more control depending on the RegexOptions you use in your Regex. For example, if you’ll never use RegexOptions.IgnorePatternWhitespace, you can remove space as a metachar.