Update italic highlighting rules to avoid intra word highlights in Markdown Mode. #4358

[RickStrahl]

*Issue #4358

This address italic highlighting in Markdown Syntax.

Description of changes:

Changes the regex to check for word boundaries or beginning/end of line endings to avoid intra-word highlighting of italic text.

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Scenarios :

*asddasd*

asdasd_asdasd_asdasdasd

asdasd *asdasd asd* asdasdasd and _asdadsasd_ asdasd


**These shouldn't hightlight**:

adsadas_asdasd_asdddasd_asdddas_adsasd _asdddasdddasd_asdddas_asdasdas_asdd 

**Underlines too**
asddasd*asdasd*asdasdasd*asdasdasd*asd* asddasddasddasdasdasd* *asdds* asdd

**Beginning of line:**

*asddasd* asdasd 

**Mix and Match:**

_asdasd asddasd_  asdasdasd _asddasd_ asdddasddd *adsasd*


*end of file*

Ironically, GitHub has the exact same issues as it highlights the text above incorrectly too 😄.

[RickStrahl]

After some additional checking it looks like there's a problem with Internet Explorer failing to render this RegEx expression causing a markdown document to fail loading. Apparently the look behind regex is not supported in IE, but works in newer browsers.

Not sure what the solution to that issue is. Perhaps conditional code with a different string for IE check.

Unfortunately IE is what I currently need this to work in as this is a desktop application with an embedded WebBrowser control. 😏

[RickStrahl]

So one of the Markdown tests is failing after this change. I've tried to figure out what that particular test checks for, but I can't see what the token / value combo accomplishes...

This change will change behavior in some cases, but legit italics should still work as before. Unfortunately I can't tell what the actual values are that are being tested in the failing test.

Maybe somebody more familiar with the testing of markdown formatting can take a look at the failing italics test.