Wayne L Waag - 5 months ago 45

C# Question

The host program I am using produces a string timecode as follows:

`00:00:01.523`

How can I replace leading zeros and change it to the following:

`--:--:-1.523`

Answer

You can use the Regex.Replace method with a *zero-width negative lookbehind assertion*:

```
string input = "00:00:01.523";
string output = Regex.Replace(input, "(?<![1-9].*)0", "-");
// Output: "--:--:-1.523"
```

The regular expression `(?<![1-9].*)0`

has two main parts:

`(?<![1-9].*)`

is a zero-width negative lookbehind assertion. It verifies that the input string does*not*match`[1-9].*`

to the left of the current position. (The regular expression`[1-9].*`

matches any digit 1–9, followed by an arbitrary string.)`0`

matches the digit 0.

Putting these two parts together, the regular expression as a whole matches any digit 0 that is *not* preceded by any digit 1–9 — namely, leading zeroes.