More conservative strong/em markdown matcher

[mrdomino]

Both matchers are now constrained so that they will only be transformed if the left-hand characters are the start of the text or preceeded by whitespace, and the right-hand characters are the end of the text or followed by whitespace or a punctuation mark.

This misses some cases, e.g. emphasis inside of parantheticals, but has better behavior with e.g. underscores in function names outside of code blocks.

Fixes #317.

[evelynmitchell]

Can you explain in a bit more detail what you intend with the change from \1 to \2 in lines 1124 and 1125?

Thanks!

[mrdomino]

It's different regex capture groups. \1 is a backreference (it says "match another instance of whatever you matched in group 1 here") and $2 is saying "put whatever you found in group 2 here" in the replacement.

In the old regex, group 1 was the opening delimiter to be matched against when searching for a closing delimiter, and group 2 was the text between the pair of delimiters. In the new regex, group 1 is the opening whitespace character, group 2 is the opening delimiter, group 3 is the text between the delimiter pair, and group 4 is the closing whitespace/punctuation. So \2 is now the way to refer to the opening delimiter for the backreference, and $1 and $4 need to be included in the replacement.

[jart]

Thank you!