ArgumentOutOfRangeException in the markdown parser with "r/Steam: www."

[Sergio0694]

I'm submitting a...

• Bug report (I searched for similar issues and did not find one)

Current behavior

The MarkdownTextBlock throws an ArgumentOutOfRange exception when entering this string:

r/Steam: www.

NOTE: the issue is present even if the address is just "ww.", or with full URLs. Eg. "www.google.com" crashes as well, if the string starts with that "r/Steam: " prefix.

The crash happens exactly here: https://github.com/windows-toolkit/WindowsCommunityToolkit/blob/3b5a1f480d65c649a1732a8b8a11866ed3c08836/Microsoft.Toolkit.Parsers/Markdown/Inlines/TextRunInline.cs#L384

In particular, with the string mentioned in the title, it happens because the end parameter is 7, while the start parameter is 2, so the resulting length is negative.

Expected behavior

The markdown should be parsed correctly as usual.

Minimal reproduction of the problem with instructions

• Create a new UWP app
• Paste this code:

  <StackPanel>
  <TextBox x:Name="SomeBox"/>
  <controls:MarkdownTextBlock Text="{Binding ElementName=SomeBox, Path=Text, UpdateSourceTrigger=PropertyChanged}"/>
  </StackPanel>

• Start the app
• Type "r/Steam: ww." in the TextBox

Environment

Nuget Package(s): `[...].Uwp.UI.Controls`

Package Version(s): `5.1.1`

Windows 10 Build Number:
- [] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [X] October 2018 Update (17763)
- [X] Insider Build (build number: 18362.x)

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [X] October 2018 Update (17763)
- [ ] Insider Build (xxxxx)

Device form factor:
- [X] Desktop
- [ ] Mobile
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio 
- [ ] 2017 (version: )
- [ ] 2017 Preview (version: )
- [X] 2019 Community

[michael-hawker]

@WilliamABradley have time to take a look?