However, since the regex must be able to match whitespace-only strings, the entire capturing group is made optional by adding a trailing question mark quantifier.

然而，由于正则表达式必需能够匹配全部由空格组成的字符串，整个捕获组通过尾随一个？量词而成为可选组。

youdao

However, since the regex must be able to match whitespace-only strings, the entire capturing group is made optional by adding a trailing question mark quantifier.

然而，由于正则表达式必需能够匹配全部由空格组成的字符串，整个捕获组通过尾随一个？量词而成为可选组。

youdao