Question: I need to filter a list of files. Some of these are csv files, and of those, some of them are appended with a control tag, ".cntl".

Example: file1.csv, file1.csv.cntl

I would like to set up a regex that checks to see if a file contains "csv" and NOT "cntl". Right now I've got this.

```
csv(?!cntl)

```

This is not working. What would a proper regex be?

PS. This is all done in C#.

Answer: 
The following is the pattern you want.

```
@"csv(?!\.cntl)"

```

But, wouldn't it be easier to check:

```
if (string.EndsWith("cntl"))

```

Using a regex is unnecessarily complicated.
This should work:

```
csv(?!.*cntl)
```