我后来通过挖掘PHP Markdown Extra找到了答案。
protected $em_relist = array(
'' => '(?:(?<!\*)\*(?!\*)|(?<!_)_(?!_))(?=\S|$)(?![\.,:;]\s)',
'*' => '(?<=\S|^)(?<!\*)\*(?!\*)',
'_' => '(?<=\S|^)(?<!_)_(?!_)',
);
protected $strong_relist = array(
'' => '(?:(?<!\*)\*\*(?!\*)|(?<!_)__(?!_))(?=\S|$)(?![\.,:;]\s)',
'**' => '(?<=\S|^)(?<!\*)\*\*(?!\*)',
'__' => '(?<=\S|^)(?<!_)__(?!_)',
);
protected $em_strong_relist = array(
'' => '(?:(?<!\*)\*\*\*(?!\*)|(?<!_)___(?!_))(?=\S|$)(?![\.,:;]\s)',
'***' => '(?<=\S|^)(?<!\*)\*\*\*(?!\*)',
'___' => '(?<=\S|^)(?<!_)___(?!_)',
);
这样:
protected $em_relist = array(
'' => '(?:(?<!\*)\*(?!\*)|(?<![a-zA-Z0-9_])_(?!_))(?=\S|$)(?![\.,:;]\s)',
'*' => '(?<=\S|^)(?<!\*)\*(?!\*)',
'_' => '(?<=\S|^)(?<!_)_(?![a-zA-Z0-9_])',
);
protected $strong_relist = array(
'' => '(?:(?<!\*)\*\*(?!\*)|(?<![a-zA-Z0-9_])__(?!_))(?=\S|$)(?![\.,:;]\s)',
'**' => '(?<=\S|^)(?<!\*)\*\*(?!\*)',
'__' => '(?<=\S|^)(?<!_)__(?![a-zA-Z0-9_])',
);
protected $em_strong_relist = array(
'' => '(?:(?<!\*)\*\*\*(?!\*)|(?<![a-zA-Z0-9_])___(?!_))(?=\S|$)(?![\.,:;]\s)',
'***' => '(?<=\S|^)(?<!\*)\*\*\*(?!\*)',
'___' => '(?<=\S|^)(?<!_)___(?![a-zA-Z0-9_])',
);
做这项工作完美。