# Regex for finding text which is not preceded by another piece of text

One of the common searches I often want to make using regex is to find a piece of text which is not preceded by another piece of text. For example, earlier today I was searching a C# file for uses of `Token` but I did not want to find `CancellationToken` in my results.

The regex for this is very simple:

```regex
(?<!Cancellation)Token
```

Given I've already stated what this regex does, you can probably instinctively see how it's structured - I'm matching the bit outside the brackets (`Token`) if it's not immediately preceded by the bit inside the brackets (`Cancellation`). But how does this work?

`?<!` may look confusing but it is the simple key to the whole problem. This creates a negative lookbehind - a group which must not match before the main expression. In other words, this is a feature of regex which solves my problem exactly!

If you haven't used regex before and want to get started, I have previously written an article which starts from scratch and teaches you enough basic regex that you can easily start getting benefit from it:

%[https://tjhilton.hashnode.dev/just-enough-regex-to-get-going]