Why does the capture group in a regular expression return the last matching character?

Detailed explanation of the return value of regular expression capture group

This article analyzes the return value of regular expression capture groups and explains a common misunderstanding.

Problem: Capturing group return value is not always expected

Consider the following regular expressions and string matching:

 /#\/((\w) )/.exec("/a-web/#/abc?");

Execution results:

 ['#/abc', 'abc', 'c', index: 7, input: '/a-web/#/abc?', groups: undefined]

The third element of the resulting array is 'c' , not the expected 'abc' , raising questions about the capture group return value.

Analysis: Features of nested capture groups

Regular expression /#\/((\w) )/ contains two nested capture groups:

1. (\w ) : Match one or more word characters.
2. ((\w) ) : Contains the first capture group, matching one or more word characters.

The return value array of exec() method contains:

• The first element: the entire matching string ( #/abc ).
• The second element: the matching result of the first capture group (\w ) ( abc ).
• The third element: the matching result of the second capture group ((\w) ) ( c ).

The key is the second capture group. It does not match the entire (\w ) result, but only captures the last matched characters (\w ) . Since \w matches abc , but (\w ) is a duplicate match, only the last matched character c is finally returned.

Conclusion: Understanding the subtleties of nested capture groups

The occurrence of 'c' is not an error, but is caused by the characteristics of nested capture groups. Understanding this feature is essential for the correct use and understanding of capture groups of regular expressions. If you need to capture the entire string, you should avoid this nested structure or use different regular expressions. For example, if you want to capture abc , you can use /#\/(\w )/ .

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