Go regexp's $1_ is parsed as a reference to a named group "1_" (underscores are
valid identifier chars), not $1 followed by literal underscore. This dropped the
captured italic content, turning <i>foo</i> into "__" instead of "_foo_".
Use ${1}_ with explicit braces to delimit the group reference, and update the
format_extended_test suite to assert the full round-trip for <i> and <em>.