边界

量词

到目前为止，我们只关心在特定输入字符串中的某个位置是否找到了匹配项。我们从不在乎匹配项在字符串中的哪个位置。

您可以通过使用边界匹配器指定此类信息来使模式匹配更精确。例如，您可能希望找到一个特定的单词，但前提是它出现在行的开头或结尾。或者您可能想知道匹配项是否发生在单词边界上，或者发生在先前匹配项的结尾。

下表列出了所有边界匹配器并对其进行了说明。

\z

Enter your regex: ^dog$
Enter input string to search: dog
I found the text "dog" starting at index 0 and ending at index 3.

Enter your regex: ^dog$
Enter input string to search:       dog
No match found.

Enter your regex: \s*dog$
Enter input string to search:             dog
I found the text "            dog" starting at index 0 and ending at index 15.

Enter your regex: ^dog\w*
Enter input string to search: dogblahblah
I found the text "dogblahblah" starting at index 0 and ending at index 11.

输入的结尾

以下示例演示了边界匹配器^和$的使用。如上所述，^匹配行的开头，$匹配结尾。

Enter your regex: \bdog\b
Enter input string to search: The dog plays in the yard.
I found the text "dog" starting at index 4 and ending at index 7.

Enter your regex: \bdog\b
Enter input string to search: The doggie plays in the yard.
No match found.

第一个示例成功，因为模式占据了整个输入字符串。第二个示例失败，因为输入字符串在开头包含额外的空格。第三个示例指定了一个表达式，该表达式允许无限的空格，然后是行末的“dog”。第四个示例要求“dog”出现在行的开头，后面跟着无限数量的单词字符。

Enter your regex: \bdog\B
Enter input string to search: The dog plays in the yard.
No match found.

Enter your regex: \bdog\B
Enter input string to search: The doggie plays in the yard.
I found the text "dog" starting at index 4 and ending at index 7.

要检查模式是否以单词边界开头和结尾（而不是更长字符串中的子字符串），只需在两侧使用\b；例如，\bdog\b。

Enter your regex: dog 
Enter input string to search: dog dog
I found the text "dog" starting at index 0 and ending at index 3.
I found the text "dog" starting at index 4 and ending at index 7.

Enter your regex: \Gdog 
Enter input string to search: dog dog
I found the text "dog" starting at index 0 and ending at index 3.

要将表达式匹配到非单词边界，请使用\B