If no match is found, return null to the client application.
如果没有发现匹配，就向客户端应用程序返回空值。
If no match is found within the input, a null value is returned.
如果在输入中未找到匹配项，则返回空值。
If no match is found return null to the client application.
如果没有发现匹配，返回null到客户端应用程序。
Every match sets a field to the captured result; every miss is set to null.
每个匹配都会把捕获结果设置到到对应的字段去；要是没有捕捉到就设为null。
Every match sets a field to the captured result; every miss is set to null.
每个匹配都会把捕获结果设置到到对应的字段去；要是没有捕捉到就设为null。
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