Bold and Italic

*italic*  or  _italic_     -> <em>italic</em>
**bold**  or  __bold__     -> <strong>bold</strong>
***bold italic***          -> <strong><em>bold italic</em></strong>

Tricky cases:

Surrounded by space? * not italic * — must touch text on both sides.

Mid-word? un*believ*able — works with * (asterisk-style emphasis is more permissive). But un_believ_able does NOT work — underscores require word boundaries.

This intentional asymmetry: prevents accidental emphasis in code-like identifiers (some_long_name).

Nesting: *outer **inner** outer* mostly works but parser-dependent.

Implementation typically uses regex with lookarounds:

Real CommonMark spec is much more nuanced — gets emphasis right in 100+ edge cases.