.NET Regex for "not this string"

Question

I'm a regex newbie and need a single expression that: matches the "an" and the "AN" but not the "and" or "AND" and matches the "o" and the "O" but not the "or" or "OR" in this predicate: 1and(2or3)AND(4OR5)an(6o7)AN(8O9) Basically I can't figure out how to convert the expression: ``` var myRegEx = Regex("[0-9 ()]|AND|OR") ``` into a "everything but", case insensitive expression. Can't use the regex word boundaries feature because the predicate isn't required to have spaces. (Added after two answers were already provided): I also need to know the index of the match, which is why I'm assuming I need to use the Regex.Match() method. Thanks! Here's what I ended up with: ``` private bool mValidateCharacters() { const string legalsPattern = @"[\d ()]|AND|OR"; const string splitPattern = "(" + legalsPattern + ")"; int position = 0; string[] tokens = Regex.Split(txtTemplate.Text, splitPattern, RegexOptions.IgnoreCase); // Array contains every legal operator/symbol found in the entry field //…

Accepted Answer

Randal Schwartz's rule: Use capturing in `Regex.Match` when you know what you want to keep, and use `Regex.Split` when you know what you want to throw away.

You wrote you want “everything but,” so

```
var input = "1and(2or3)AND(4OR5)an(6o7)AN(8O9)";
foreach (var s in Regex.Split(input, @"[\d()]|AND|OR", RegexOptions.IgnoreCase))
  if (s.Length > 0)
    Console.WriteLine("[{0}]", s);
```

Output:

```
[an]
[o]
[AN]
[O]
```

To get the offsets, save the separators by enclosing the regular expression in parentheses:

```
var input = "1and(2or3)AND(4OR5)an(6o7)AN(8O9)";
string pattern = @"([\d()]|AND|OR)";
int offset = 0;
foreach (var s in Regex.Split(input, pattern, RegexOptions.IgnoreCase)) {
  if (s.ToLower() == "an" || s.ToLower() == "o")
    Console.WriteLine("Found [{0}] at offset {1}", s, offset);
  offset += s.Length;
}
```

Output:

```
Found [an] at offset 19
Found [o] at offset 23
Found [AN] at offset 26
Found [O] at offset 30
```

.NET Regex for "not this string"

Solution

Problem